Question 1

In elementary physics, friction between two solid surfaces in contact mainly arises due to which microscopic feature present on the surfaces?

Accepted Answer

Introduction / Context: This question focuses on the basic cause of friction between two solid surfaces in contact. Friction is a resistive force that opposes relative motion or the tendency of motion. Although surfaces may look smooth to the naked eye, at the microscopic level they are far from perfectly smooth. Understanding what really causes friction helps in explaining why lubrication works and why friction changes with surface treatment. Given Data / Assumptions: Two solid surfaces are in contact and may be trying to slide over one another. The question asks what property of these surfaces causes friction. Options include irregularities, smoothness, densities and gaps. We assume normal conditions, not exotic materials such as perfectly smooth atomic surfaces. Concept / Approach: Real surfaces, even polished ones, have microscopic hills and valleys called irregularities or asperities. When two surfaces are pressed together, these irregularities interlock. To start or maintain relative motion, these interlocking points must be lifted, broken or deformed, which requires force. This resistance appears as friction. Greater roughness usually means more pronounced irregularities and higher friction. Smoothness actually reduces friction, while density and simple gaps are not the direct microscopic cause described in basic physics courses. Step-by-Step Solution: Step 1: Visualise a surface under a microscope. Even a surface that looks shiny will show tiny peaks and valleys. Step 2: When two such surfaces touch, the high points or asperities of one surface press against those of the other surface. Step 3: Under the action of a normal force, these asperities can get locked together or experience strong contact forces. Step 4: When we try to move one surface relative to the other, we must overcome this interlocking, so a tangential resistive force appears. Step 5: This resistive force is called friction and is fundamentally caused by these microscopic irregularities and, in more advanced treatments, also by molecular forces at contact points. Step 6: Therefore, the correct option that captures the basic school level explanation is irregularities on the surfaces. Verification / Alternative check: If we rub two very rough surfaces together, friction is large and movement is difficult. If we polish or smooth the surfaces, friction decreases. This direct observation supports the idea that it is the surface irregularities that largely control friction. Introducing lubricants fills in the microscopic valleys and separates the surfaces with a thin fluid layer, effectively reducing the impact of asperities and decreasing friction further. These everyday examples agree with the explanation that irregularities cause friction. Why Other Options Are Wrong: Smoothness tends to reduce friction rather than cause it; smoother surfaces have fewer and smaller irregularities. Densities of materials influence mass and inertia but are not the primary microscopic reason for friction between surfaces. Gaps between surfaces may exist, but friction arises where actual contact occurs, not in the empty spaces. Common Pitfalls: Some learners think friction is a mysterious property unrelated to surface structure, or that only rough looking surfaces have friction. In reality, even highly polished surfaces have microscopic irregularities and also experience molecular attraction at contact points. At the school level, the simplest and most important idea is that friction is due to microscopic irregularities that interlock. Keeping this image in mind makes it easier to understand how polishing, lubricating or changing materials can increase or decrease friction. Final Answer: Friction between two surfaces in contact is mainly caused by microscopic irregularities on the surfaces.

Question 2

In alternating current (AC) circuit theory, the combined opposition to the flow of AC known as impedance is measured in which SI unit?

Accepted Answer

Introduction / Context: This question checks your knowledge of electrical units in AC circuit analysis. In direct current (DC) circuits we talk about resistance, but in AC circuits the opposition to current flow is more general and includes resistance, inductive reactance and capacitive reactance together. This combined effect is called impedance. Knowing the correct SI unit for impedance is essential for understanding AC circuit calculations and reading circuit diagrams and specifications. Given Data / Assumptions: We are dealing with AC circuits where impedance is defined. Impedance is the generalised opposition to alternating current. Options for the unit include ohm, henry, tesla and hertz. We assume standard SI units are used with no special derived units beyond the usual ones. Concept / Approach: Impedance is usually denoted by Z and is a complex quantity that combines resistance R and reactance X. However, its unit is the same as that of resistance because both represent an opposition to current. Resistance is measured in ohm, symbol Ω. Therefore impedance is also measured in ohm. The henry is the unit of inductance, tesla is the unit of magnetic flux density and hertz is the unit of frequency. They do not measure impedance directly. So the correct unit is ohm. Step-by-Step Solution: Step 1: Recall that in DC circuits, resistance R is the opposition to current and is measured in ohm. Step 2: In AC circuits, the total opposition to current includes resistance and reactance and is called impedance Z. Step 3: Even though impedance can be represented as a complex number (R + jX), the magnitude of impedance still has the same unit as resistance. Step 4: Therefore, the SI unit of impedance is the ohm (Ω). Step 5: Henry (H) measures inductance, which describes how a coil opposes changes in current, not the overall impedance itself. Step 6: Tesla (T) measures magnetic flux density in electromagnetism, not opposition to current. Step 7: Hertz (Hz) measures frequency and does not represent impedance. Verification / Alternative check: In practical AC circuit problems, you often use formulas such as Z = V / I, where V is voltage in volt and I is current in ampere. This is similar to Ohm law R = V / I. Since volt per ampere defines the ohm, and Z is computed by the same ratio, impedance must also be expressed in ohm. Component datasheets for resistors, inductors and capacitors in AC circuits usually specify impedance magnitude in ohm at a given frequency, further confirming this unit choice. Why Other Options Are Wrong: Henry is the unit of inductance, which is a property of coils and describes induced emf, not the overall impedance. Tesla is used for magnetic flux density and is unrelated to direct measurement of circuit opposition to current. Hertz is the unit of frequency, describing how many cycles occur per second, not the opposition to current. Common Pitfalls: Some students may associate impedance mainly with inductors and capacitors and mistakenly choose henry as the unit. It is important to remember that while inductance and capacitance contribute to impedance, their own units are henry and farad, not ohm. Impedance itself, as the combined effect of resistance and reactance, has the same unit as resistance. Keeping the distinction clear between physical quantity and its contribution to impedance prevents confusion. Final Answer: Impedance in AC circuits is measured in the SI unit called the ohm.

Question 3

In thermodynamics, entropy is a state function that measures disorder and energy unavailability. What is the SI unit of entropy?

Accepted Answer

Introduction / Context: This question tests your knowledge of the correct SI unit for entropy, an important concept in thermodynamics. Entropy is associated with the degree of randomness or disorder in a system and with the amount of energy that is not available to do useful work. Understanding its unit is important when using thermodynamic equations such as those for reversible processes and for changes in entropy in heat transfer problems. Given Data / Assumptions: We consider entropy as defined in classical thermodynamics. We work with the SI (International System of Units). Options given include watt per kelvin, newton per kelvin, pascal per kelvin and joule per kelvin. No advanced statistical mechanics framework is required, only the standard engineering definition. Concept / Approach: Entropy change dS for a reversible process is defined by the relation dS = dQ_rev / T, where dQ_rev is the reversible heat transfer and T is absolute temperature. In SI units, heat dQ_rev is measured in joule and temperature T is measured in kelvin. Therefore, entropy and entropy change have the unit joule per kelvin (J/K). The other units listed arise from force, power or pressure relationships and do not match this ratio of energy to temperature. Step-by-Step Solution: Step 1: Recall the thermodynamic definition for reversible processes: dS = dQ_rev / T. Step 2: Identify the units: dQ_rev is heat transfer measured in joule (J). Step 3: Temperature T is measured in kelvin (K) in the SI system. Step 4: So the unit of dS is (joule) / (kelvin) or J/K. Step 5: Recognise that this unit applies to entropy S itself and to its changes. Step 6: Compare with the given options and see that joule per kelvin is present as one of them. Step 7: Conclude that the correct SI unit of entropy is joule per kelvin. Verification / Alternative check: Textbooks on thermodynamics and engineering handbooks list entropy values such as specific entropy s in units of J/(kg K) and total entropy S in J/K. These consistent usages confirm that joule per kelvin is the accepted SI unit. Also, when applying the second law of thermodynamics to cycles or processes, entropy changes are always expressed with energy in the numerator and temperature in the denominator, reinforcing the J/K unit structure. Why Other Options Are Wrong: Watt per kelvin combines power (J/s) and temperature, and is used in some heat transfer coefficients, not as the unit of entropy itself. Newton per kelvin mixes force and temperature and has no standard role as a unit of entropy. Pascal per kelvin involves pressure per temperature and is not used for entropy; it belongs to different derived quantities. Common Pitfalls: Students sometimes confuse entropy units with those of related quantities like heat transfer coefficient or specific heat capacity. Specific heat capacity, for example, has units J/(kg K), which may appear similar but include mass in the denominator. Entropy per unit mass does share this same form, so it is important to distinguish between total entropy (J/K) and specific entropy (J/(kg K)). Focusing on the basic relation dS = dQ_rev / T helps fix the correct unit firmly in memory. Final Answer: The SI unit of entropy is joule per kelvin (J/K).

Question 4

In the International System of Units (SI), what is the fundamental base unit used to measure the amount of substance in chemistry and physics?

Accepted Answer

Introduction / Context: This general knowledge question checks whether you know the SI base units, in particular the unit for amount of substance. In chemistry and physics, we often talk about how many particles, atoms or molecules take part in a reaction. Directly counting them is impossible, so scientists use a convenient counting unit. Recognising this unit is essential for understanding chemical equations, gas laws and many physical calculations. Given Data / Assumptions: The context is the SI system of units. We are concerned with the base quantity called amount of substance. Options given are Mole, Candela, Kelvin and Meter. We assume standard scientific definitions, not informal usage. Concept / Approach: The SI base unit for amount of substance is the mole, symbol mol. One mole corresponds to a fixed number of specified elementary entities, such as atoms or molecules, equal to the Avogadro number. Other quantities listed in the options are SI base units for different physical quantities: candela for luminous intensity, kelvin for temperature and metre for length. Only mole corresponds to amount of substance. Step-by-Step Solution: Step 1: Recall the seven SI base quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity. Step 2: Match each quantity with its base unit: metre for length, kilogram for mass, second for time, ampere for current, kelvin for temperature, mole for amount of substance and candela for luminous intensity. Step 3: Identify that the question asks specifically for the unit of amount of substance. Step 4: From the list above, the unit for amount of substance is mole. Step 5: Check the options and see that Mole appears as option A. Step 6: Confirm that candela, kelvin and metre correspond to other base quantities, not amount of substance. Step 7: Conclude that Mole is the correct answer. Verification / Alternative check: In chemical equations, you often read statements such as one mole of hydrogen reacts with one mole of chlorine to produce two moles of hydrogen chloride. Molar mass, molar concentration and molar volume all use the unit mole. Standard references and periodic tables quote atomic and molecular masses in grams per mole. All these examples consistently use mole as the base counting unit for amount of substance, confirming that mole is the correct SI base unit. Why Other Options Are Wrong: Candela is the SI base unit for luminous intensity, describing how bright a light source is in a given direction. Kelvin is the SI base unit for thermodynamic temperature, used in temperature scales and thermodynamic relations. Meter is the SI base unit for length, used to measure distance, not the amount of substance. Common Pitfalls: Sometimes learners mix up the meanings of SI base units because they memorise the list without linking each unit to the corresponding physical quantity. It is helpful to associate each unit with a clear example: metre with distance, kelvin with absolute temperature, candela with light from a lamp and mole with counting atoms and molecules. This association makes it easier to recall that the amount of substance uses the unit mole, a central idea in chemistry and physics. Final Answer: The fundamental SI base unit for amount of substance is the Mole (mol).

Question 5

Which of the following statements about a photovoltaic (solar) cell is incorrect with reference to how it works and what it is used for?

Accepted Answer

Introduction / Context: This question tests your conceptual understanding of what a photovoltaic cell is and how it functions. A photovoltaic cell is commonly known as a solar cell, and it is widely used to convert light energy from the Sun or other sources into electrical energy. Knowing which statements correctly describe a photovoltaic cell and which confuse it with other devices, such as light emitting diodes, is important for basic physics and general science awareness. Given Data / Assumptions: We are dealing with a photovoltaic cell, commonly called a solar cell. The options mention names, detection of infrared radiation, energy storage and conversion of electric energy into light. We assume standard devices used in solar panels and photodetectors. Only one statement is meant to be incorrect, and we must identify it. Concept / Approach: A photovoltaic cell is a semiconductor device that converts light energy directly into electrical energy using the photovoltaic effect. It is indeed referred to as a solar cell when used with sunlight. Many photodiodes and photovoltaic cells are sensitive in the infrared region and can act as detectors for that radiation. However, a single photovoltaic cell does not act as an energy storage device; it generates electricity only while it is illuminated, and storage requires external batteries or capacitors. Most importantly, a photovoltaic cell does not convert electric energy into light energy. That is the function of devices such as light emitting diodes and lamps. Therefore the statement that it converts electric energy into light energy is incorrect for a photovoltaic cell. Step-by-Step Solution: Step 1: Recognise that photovoltaic cell and solar cell refer to the same basic device that converts light to electricity, so option A is correct. Step 2: Note that some photovoltaic and photodiode devices are designed to respond to infrared radiation as detectors, so option B can be considered correct in general scientific usage. Step 3: Understand that a photovoltaic cell produces electric energy only as long as light falls on it and does not store energy by itself; storage requires separate components, so stating that it cannot by itself store energy is correct, making option C a true statement. Step 4: Examine option D, which says that a photovoltaic cell converts electric energy into light energy. This is opposite of what a photovoltaic cell does and actually describes the behaviour of a light emitting diode or electric lamp. Step 5: Since a photovoltaic cell converts light energy into electrical energy, not the other way round, option D is incorrect. Step 6: Conclude that option D is the only incorrect statement about a photovoltaic cell. Verification / Alternative check: Consider a solar panel on a rooftop. When sunlight falls on it, it generates electric power that is then used to run loads or charge batteries. At night, when there is no light, the panel does not emit light or store energy internally; it simply remains inactive. This real world behaviour clearly matches the description of converting light into electrical energy and needing external storage systems. The panel does not glow or produce light when electricity is supplied to it, which confirms that converting electric energy into light is not its function. Why Other Options Are Wrong or Right: Option A is correct because the term photovoltaic cell is commonly used interchangeably with solar cell. Option B is correct because suitably designed photovoltaic and photodiode devices can act as infrared detectors in various instruments. Option C is correct because a photovoltaic cell generates but does not store energy; energy storage requires separate batteries or capacitors. Option D is incorrect because converting electric energy into light energy is the role of light emitting devices, not photovoltaic cells. Common Pitfalls: Students sometimes confuse photovoltaic cells with light emitting diodes or think that any device involving light and electricity can work in both directions. In reality, while some devices can operate reversibly under special conditions, the standard function of a photovoltaic cell is to convert light into electrical energy only. It is also easy to assume that because solar systems involve storage, the cell itself stores energy, which is not true. Remember that generation, detection and storage are different roles usually handled by different components. Final Answer: The incorrect statement is that a photovoltaic cell converts electric energy into light energy.

Question 6

In basic mechanics, the rate at which work is done or energy is transferred per unit time is called which physical quantity?

Accepted Answer

Introduction / Context: This question is about the definition of power in physics. Work and energy tell us how much change has occurred, but power tells us how fast that change takes place. Two machines may do the same amount of work, but the one that does it in less time has greater power. Understanding this distinction is essential in mechanics, engineering and everyday life where ratings of devices are often given in watts or horsepower. Given Data / Assumptions: The quantity described is the rate of doing work. Equivalently, it is the rate of transfer of energy. We are asked to name the physical quantity, not its unit. Options are energy, power, momentum and impulse. Concept / Approach: Power is defined in physics as work done per unit time or energy transferred per unit time. In symbol form, P = W / t, where P is power, W is work and t is time. Energy is the capacity to do work, while momentum is the product of mass and velocity, and impulse is change in momentum. Therefore, among the options, power is the correct name for the rate of doing work. Step-by-Step Solution: Step 1: Recall the definition of work: work is done when a force causes displacement in the direction of the force. Step 2: Consider that the same amount of work can be done quickly or slowly, which motivates the need for a separate quantity that measures how fast work is done. Step 3: Define power P as P = W / t, where W is work in joule and t is time in second. Step 4: Recognise that this definition clearly states that power is the rate of doing work. Step 5: Note that energy is a broader concept representing the capacity to perform work and is not itself a rate. Step 6: Observe that momentum (m * v) and impulse (change in momentum) are related to motion and collisions, not directly to the rate of work. Step 7: Conclude that the correct term for the rate of doing work is power. Verification / Alternative check: In everyday life, we see power ratings on electrical appliances, such as a 60 watt bulb or a 1000 watt heater. These ratings tell us how much energy the device uses each second, since 1 watt equals 1 joule per second. A 1000 watt heater uses energy faster than a 100 watt bulb. This practical interpretation matches the idea that power measures how quickly work or energy transfer occurs, reinforcing that power is the correct term. Why Other Options Are Wrong: Energy describes how much work a body can do but does not say how quickly the work is done. Momentum is the product of mass and velocity and describes the state of motion, not the rate of energy transfer. Impulse is the change in momentum produced by a force acting for a short time and is not related to the rate of doing work. Common Pitfalls: Students sometimes loosely use the word energy to describe powerful machines, but strictly in physics, energy and power are different quantities. A car may have a certain amount of fuel energy, but its engine power determines how quickly that energy can be used to do work. Keeping the definition P = W / t in mind helps avoid confusion between these related but distinct concepts. Final Answer: The rate of doing work or transferring energy per unit time is called Power.

Question 7

In nuclear physics and archaeology, the radiocarbon dating technique is primarily used for which type of estimation?

Accepted Answer

Introduction / Context: This general knowledge question concerns radiocarbon dating, a famous technique that links nuclear physics with archaeology, geology and palaeontology. The method uses the radioactive isotope carbon 14 to determine how long it has been since an organism died. Knowing what radiocarbon dating is used for helps you connect physics concepts like radioactivity with real world applications in earth sciences and history. Given Data / Assumptions: Radiocarbon dating is the method under discussion. The sample of interest is usually organic material or fossils containing carbon. Options mention soil contamination, water content, age of fossils and soil quality. We assume standard scientific use of carbon 14 dating. Concept / Approach: Radiocarbon dating relies on the fact that carbon 14, a radioactive isotope of carbon, is continuously produced in the atmosphere and taken up by living organisms. While an organism is alive, the ratio of carbon 14 to carbon 12 in its body remains roughly constant. After death, the intake stops, and carbon 14 begins to decay with a known half life. By measuring how much carbon 14 remains in a sample and comparing it to the expected original amount, scientists can estimate the time since the organism died. Thus the technique is used to estimate the age of fossils and archaeological remains, not soil quality or water content. Step-by-Step Solution: Step 1: Recognise that radiocarbon dating is based on the radioactive isotope carbon 14. Step 2: Understand that carbon 14 is incorporated into living organisms through processes like photosynthesis and the food chain. Step 3: After an organism dies, it no longer exchanges carbon with the environment, so the carbon 14 present in its tissues begins to decay at a known rate. Step 4: By measuring the remaining carbon 14 activity in a sample, scientists can calculate how much time has passed since death. Step 5: This calculation yields an estimate of the age of the fossil or archaeological specimen. Step 6: Observe that radiocarbon dating does not directly measure soil contamination, water content or soil quality. Step 7: Therefore, among the options, the correct use is to estimate the age of fossils. Verification / Alternative check: Radiocarbon dating is widely used to date ancient wooden artefacts, bones, charcoal and other carbon containing remains up to about 50,000 years old. Famous examples include dating of ancient human skeletons, cave paintings and historic wooden structures. Scientific reports always present radiocarbon results as age estimates in years before present. Soil quality and contamination are assessed by different chemical and physical tests, not by radiocarbon dating, which confirms that age determination is the core use of this method. Why Other Options Are Wrong: Estimating soil contamination involves chemical analysis of pollutants and is not directly related to carbon 14 decay measurements. Estimating the amount of water in fossils is a different physical or chemical measurement and does not require radiocarbon dating. Estimating the quality of soil involves nutrients, texture and biological activity, not radioactive decay of carbon. Common Pitfalls: Some learners loosely associate any measurement involving radiation with contamination or pollution, which is not always the case. Radiocarbon dating is a specific application that uses a natural isotope to date organic remains. It does not measure toxicity or water content. Remembering the key words carbon 14, half life and age of fossils will help you quickly identify the correct application of radiocarbon dating in exam questions. Final Answer: Radiocarbon dating is primarily used to estimate the age of fossils and other ancient carbon containing remains.

Question 8

For pure water at standard atmospheric pressure, at which temperature in degree Celsius is its density maximum?

Accepted Answer

Introduction / Context: This question is from basic physics and deals with an unusual but very important property of water. Most substances become denser as they cool, but water behaves differently near its freezing point. Its density reaches a maximum at a temperature slightly above the freezing point. This anomaly has significant implications for lakes, rivers and aquatic life in cold climates. Given Data / Assumptions: The substance is pure water. Pressure is assumed to be approximately one atmosphere. The temperature range of interest is near the freezing point: from 0 degree Celsius up to about 12 degree Celsius. We want the temperature at which the density of water reaches its maximum value. Concept / Approach: Water exhibits anomalous expansion near 4 degree Celsius. As water is cooled from higher temperatures, its density increases up to about 4 degree Celsius. Below 4 degree Celsius, water actually expands as it approaches the freezing point, so its density decreases. This means the maximum density occurs at around 4 degree Celsius, not at 0 degree Celsius where ice begins to form. This anomaly is why cold lakes have warmer, denser water at the bottom during winter. Step-by-Step Solution: Step 1: Recall the general rule that cooling a liquid usually increases its density because the molecules move less and come closer. Step 2: Recognise that water follows this pattern down to around 4 degree Celsius. Step 3: Below about 4 degree Celsius, water behaves anomalously and begins to expand on further cooling. Step 4: Expansion means that the same mass occupies a larger volume, so the density decreases. Step 5: Therefore, the density reaches a maximum at the turning point of this behaviour, which is around 4 degree Celsius. Step 6: Check the options and see that 4 degree Celsius is one of them. Step 7: Conclude that the density of water is maximum at 4 degree Celsius. Verification / Alternative check: Temperature density tables for water show that at 0 degree Celsius, water and ice coexist and the density of liquid water is slightly less than at 4 degree Celsius. As temperature rises from 0 to 4 degree Celsius, density increases; above 4 degree Celsius, density decreases gradually again with further heating. This symmetric behaviour confirms that 4 degree Celsius is the point of maximum density. Observations of lakes in winter, where water at 4 degree Celsius sinks to the bottom, also support this property. Why Other Options Are Wrong: 12 degree Celsius is warmer and water at this temperature is less dense than at 4 degree Celsius. 8 degree Celsius is still above the temperature of maximum density; density at 8 degree Celsius is lower than at 4 degree Celsius. 0 degree Celsius is the freezing point of water, but due to anomalous expansion, water at this temperature is less dense than at 4 degree Celsius. Common Pitfalls: Many students assume that the maximum density must occur at the freezing point or at the lowest temperature before freezing, but water is an exception. It is important to memorise that water has its maximum density at about 4 degree Celsius. This single fact explains why ice floats and why ponds freeze from the surface downwards, allowing fish to survive in liquid water below the surface ice during winter. Final Answer: The density of pure water is maximum at 4 degree Celsius.

Question 9

During uniform motion of an object along a straight line, which physical quantity describing the motion remains constant with time?

Accepted Answer

Introduction / Context: This question examines your understanding of uniform motion along a straight line, a basic idea in kinematics. Uniform motion means that the object covers equal distances in equal intervals of time in a fixed direction. Identifying which quantity remains constant in such motion helps you distinguish between speed, velocity, distance and acceleration in one dimensional motion problems. Given Data / Assumptions: An object is moving in a straight line. The motion is uniform, meaning there is no speeding up or slowing down. We assume a fixed reference frame. We must choose which listed quantity stays constant with time. Concept / Approach: Uniform motion along a straight line means constant velocity. Velocity is a vector that has both magnitude (speed) and direction. In uniform straight line motion, both the speed and the direction remain unchanged, so velocity is constant. Distance from the starting point increases steadily with time and does not remain constant. Time itself always increases. Acceleration is the rate of change of velocity; if velocity is constant, acceleration is zero, but zero is a constant value. However, when the question asks which quantity remains constant describing the state of motion, velocity being constant is the key feature of uniform motion. Step-by-Step Solution: Step 1: Define uniform motion as motion in which equal distances are covered in equal time intervals in a given direction. Step 2: Recognise that this implies constant speed and a fixed direction of motion. Step 3: Combine constant speed and fixed direction to conclude that velocity, a vector with magnitude and direction, remains constant. Step 4: Understand that as time passes, the distance travelled from the starting point increases steadily, so distance does not remain constant. Step 5: Recall that acceleration is defined as the rate of change of velocity. For uniform motion, velocity does not change, so acceleration is zero throughout, but the important description of the motion is that velocity is constant. Step 6: Note that time is never constant; it always moves forward and increases. Step 7: Therefore, among the options, velocity is the quantity that remains constant with time in uniform straight line motion. Verification / Alternative check: Consider a car moving on a straight road with its speedometer fixed at 60 km/h and direction unchanged. Over each hour, it covers 60 kilometres, and its velocity vector remains the same in magnitude and direction. The distance from the starting point after 1 hour, 2 hours and 3 hours is 60, 120 and 180 kilometres respectively, which clearly changes. Time also advances. Acceleration remains zero, but when we define the motion as uniform, we focus on the constancy of velocity. This everyday example confirms the reasoning. Why Other Options Are Wrong: Time cannot remain constant because it continuously increases as motion proceeds. Acceleration is zero in uniform motion, but the question points to the characteristic of the motion itself, which is constant velocity; marking acceleration as constant can be misleading in this context. Distance increases steadily as the object moves, so it does not remain constant with time. Common Pitfalls: Some students confuse uniform motion with rest and think distance is constant. However, in uniform motion, the object is definitely moving, so distance from the start changes. Others focus on acceleration being zero and may select acceleration as the answer. While acceleration is indeed zero, the defining property of uniform straight line motion is that velocity is constant. Remembering that uniform motion means constant velocity helps you select the right option quickly. Final Answer: In uniform motion along a straight line, the quantity that remains constant with time is the Velocity of the object.

Question 10

Which meteorological instrument is specifically used for measuring the atmospheric air pressure at a place?

Accepted Answer

Introduction / Context: This question is from basic meteorology and physics and asks about instruments used to measure different atmospheric quantities. Weather stations and laboratories use special devices to track air pressure, temperature, humidity and wind speed. Knowing which instrument corresponds to air pressure helps you interpret weather reports and understand how meteorologists predict weather changes. Given Data / Assumptions: We want the name of the instrument that measures atmospheric air pressure. Options include anemometer, barometer, hygrometer and thermometer. We assume standard meteorological instruments. Concept / Approach: A barometer is the instrument used to measure atmospheric pressure. It may be a mercury barometer, an aneroid barometer or a digital pressure sensor, but the general name remains barometer. An anemometer measures wind speed, a hygrometer measures humidity and a thermometer measures temperature. Therefore, the correct instrument for air pressure is the barometer. Step-by-Step Solution: Step 1: Recall that air pressure is the force exerted by the weight of the air column per unit area on the Earth surface. Step 2: Recognise that barometers were historically invented to measure this pressure using mercury columns. Step 3: Modern barometers include aneroid barometers and electronic barometric sensors, but they still retain the same name. Step 4: Anemometers, by contrast, are designed with cups or vanes to measure wind speed, not pressure. Step 5: Hygrometers use materials that respond to moisture to measure relative humidity, not pressure. Step 6: Thermometers measure temperature using thermometric substances or electronic sensors, not air pressure. Step 7: Thus, the correct instrument for measuring atmospheric pressure is the barometer. Verification / Alternative check: Weather reports often show barometric pressure readings in units such as millibar or hectopascal. Devices at home or in smartphones that display atmospheric pressure are often called barometers or barometric sensors. The word baro comes from a Greek root meaning weight, reflecting that the instrument measures the weight of the air column. This consistent naming across devices confirms that barometer is the correct instrument name for air pressure measurements. Why Other Options Are Wrong: Anemometer is used for measuring wind speed, often displayed in km/h or m/s, not pressure. Hygrometer is designed to measure humidity or moisture content in the air, not the pressure. Thermometer is used for measuring temperature, usually in degree Celsius or kelvin, not air pressure. Common Pitfalls: Because all the options are meteorological instruments, students sometimes confuse their functions. A useful memory aid is to associate barometer with barometric pressure charts, anemometer with moving arms or cups that spin in the wind, hygrometer with hygro meaning moisture and thermometer with thermo meaning heat. Keeping these associations clear will help you answer instrument questions quickly and accurately in exams. Final Answer: The instrument used to measure atmospheric air pressure is the Barometer.

Question 11

For a wave such as a sound wave or light wave, the intensity of the wave is proportional to which mathematical function of its amplitude?

Accepted Answer

Introduction / Context: This question is about the relationship between the amplitude of a wave and its intensity. Intensity tells us how much energy a wave transports per unit area per unit time. For many types of waves, including sound waves and electromagnetic waves such as light, there is a simple mathematical relationship between amplitude and intensity. Understanding this relationship is important for explaining why small changes in amplitude can cause large changes in perceived loudness or brightness. Given Data / Assumptions: We have a wave characterised by an amplitude A. We are interested in how intensity I depends on A. Options include direct proportionality to amplitude, square of amplitude, square root of amplitude and cube of amplitude. We assume linear wave behaviour in a medium where standard wave theory applies. Concept / Approach: According to wave theory, the energy carried by a wave is proportional to the square of its amplitude. Since intensity is energy transmitted per unit area per unit time, it also becomes proportional to the square of the amplitude. For sound, doubling the amplitude increases intensity by a factor of four. For light, electric and magnetic field amplitudes lead to intensity also being proportional to the field amplitude squared. Therefore, intensity is proportional to the square of amplitude, not to the amplitude itself or other powers listed. Step-by-Step Solution: Step 1: Let the amplitude of the wave be A. This represents the maximum displacement of particles (for sound) or maximum field strength (for light). Step 2: Write the qualitative relationship from wave theory: energy carried by the wave ∝ A^2. Step 3: Recognise that intensity I is defined as energy per unit area per unit time, so I is also proportional to the energy transported by the wave. Step 4: Therefore, I ∝ A^2 for a given medium and frequency. Step 5: Translate this into words: intensity is proportional to the square of amplitude. Step 6: Compare with the options and identify square of amplitude as the correct choice. Step 7: Conclude that a small increase in amplitude leads to a larger increase in intensity because of the square relationship. Verification / Alternative check: For sound waves, decibel scales relate to the logarithm of intensity ratios. A small increase in amplitude results in a much larger increase in intensity and perceived loudness, consistent with a square dependence. For light, interference patterns depend on the square of the sum of amplitudes. When two coherent light waves interfere constructively, the resultant intensity is four times that of one wave because the amplitude doubles and intensity scales with amplitude squared. These experimental facts support the I ∝ A^2 relationship. Why Other Options Are Wrong: Direct proportionality to amplitude would underestimate the energy change when amplitude changes and does not match experimental observations. Proportionality to the square root of amplitude would make intensity change too slowly with amplitude, contradicting energy considerations. Proportionality to the cube of amplitude is not supported by basic wave theory and would make intensity grow too fast with amplitude. Common Pitfalls: Some learners confuse amplitude and intensity as if they were directly proportional. It is important to remember that intensity relates to energy, and energy for many oscillatory systems scales as amplitude squared. Keeping in mind the key relation I ∝ A^2 helps you reason correctly about changes in loudness, brightness and energy transmission whenever amplitude changes. Final Answer: The intensity of a wave is proportional to the Square of amplitude of the wave.

Question 12

Which scientific instrument is commonly used to measure the humidity, that is, the amount of water vapour present in the atmosphere?

Accepted Answer

Introduction / Context: This question is about basic weather instruments and their uses. Humidity is a measure of how much water vapour is present in the air, and it strongly influences comfort, weather patterns and processes like drying of clothes. Meteorologists and HVAC engineers need accurate humidity measurements, and they use a special instrument for this purpose. Knowing the correct name of the instrument is useful for both science and general knowledge exams. Given Data / Assumptions: The quantity to be measured is humidity or moisture content in the air. Options include barometer, anemometer, thermometer and hygrometer. We assume standard meteorological instruments and their classical definitions. Concept / Approach: A hygrometer is the instrument designed to measure humidity. It can be a simple hair hygrometer, a wet and dry bulb psychrometer or an electronic humidity sensor, but the generic name is hygrometer. A barometer measures air pressure, an anemometer measures wind speed and a thermometer measures temperature. Therefore, hygrometer is the only option that directly corresponds to humidity measurement. Step-by-Step Solution: Step 1: Recall that humidity refers to the amount of water vapour in the air and is often expressed as relative humidity or absolute humidity. Step 2: Understand that an instrument used to measure humidity is called a hygrometer (hygro relating to moisture). Step 3: Recognise that barometers are used for measuring atmospheric pressure, not humidity. Step 4: Note that anemometers measure the speed of wind and are not related to moisture measurement. Step 5: Observe that thermometers are used to measure temperature, a different physical quantity. Step 6: Compare these functions with the requirement of measuring humidity. Step 7: Conclude that the correct instrument for humidity is the hygrometer. Verification / Alternative check: Weather reports often mention relative humidity in percentage, and weather stations use hygrometers or psychrometers to record these values. Many digital room thermometers now include a humidity readout and are sometimes labelled thermo hygrometers, indicating that hygrometer is the humidity measurement component. These practical devices confirm the name and role of a hygrometer as the instrument used to measure atmospheric humidity. Why Other Options Are Wrong: Barometer measures atmospheric pressure, which is commonly expressed in millibar or hectopascal, not humidity. Anemometer measures wind speed and sometimes wind direction, not moisture content in the air. Thermometer measures temperature, usually in degree Celsius or kelvin, and does not measure water vapour content. Common Pitfalls: Because all the instruments listed are used in weather observations, it is easy to mix up their purposes. A simple memory aid is to link hygro with moisture, baro with weight or pressure, anemo with wind and thermo with heat or temperature. Remembering these prefixes makes it easier to select hygrometer whenever the question involves humidity or moisture in the air. Final Answer: The instrument used to measure atmospheric humidity is the Hygrometer.

Question 13

In a qualitative way, the tendency of an undisturbed object to remain at rest or to continue moving with the same velocity is called what?

Accepted Answer

Introduction / Context: This question comes from Newton first law of motion and asks about a key concept in mechanics. The law describes how objects behave when no net external force acts on them. The qualitative term that captures the tendency of matter to resist changes in its state of motion is very important in understanding why seat belts are needed, why passengers lurch forward when a bus stops and many other everyday phenomena. Given Data / Assumptions: The object in question is undisturbed, meaning no net external force is acting on it. We are told the object tends to stay at rest or continue moving at constant velocity. Options include force, acceleration, friction and inertia. We assume basic Newtonian mechanics applies. Concept / Approach: Newton first law states that a body continues in its state of rest or uniform motion in a straight line unless acted upon by a net external force. The property of matter that resists changes in its state of motion is called inertia. Heavier bodies usually show greater inertia and are more difficult to start or stop. Force is what changes motion, acceleration is the result of net force and friction is a specific kind of resistive force, not the general tendency to resist change. Therefore, the correct term for this tendency is inertia. Step-by-Step Solution: Step 1: State Newton first law in words: an object remains at rest or in uniform straight line motion unless a net external force acts on it. Step 2: Recognise that this behaviour reflects a built in resistance to changes in motion. Step 3: This resistance to change in the state of rest or uniform motion is given the name inertia. Step 4: Force is the external agency that can overcome inertia and change the motion, so it is not the tendency itself. Step 5: Acceleration is the change in velocity produced by a net force; it is an effect, not the tendency to resist change. Step 6: Friction is a specific force that always opposes motion or attempted motion; it is not the universal property of all matter. Step 7: Conclude that inertia is the correct term for the described tendency. Verification / Alternative check: Consider a heavy truck and a light car at rest. It is harder to start the truck moving because it has more mass and therefore more inertia. When both are moving at the same speed, it is also harder to stop the truck. Similarly, when a moving bus brakes suddenly, passengers tend to move forward due to their inertia, as their bodies try to maintain the previous state of motion. These examples confirm that inertia is the property that resists changes in motion, consistent with the definition in the question. Why Other Options Are Wrong: Force is what causes changes in motion according to Newton second law; it is not the resistance to change. Acceleration is the change in velocity per unit time and results from a net force, not the tendency to remain unchanged. Friction is a specific type of contact force that opposes relative motion between surfaces and is not a general property of matter. Common Pitfalls: Students sometimes confuse inertia with friction because both can oppose motion. However, friction is an external force between surfaces, whereas inertia is an inherent property of mass. In a frictionless environment, inertia still exists and objects continue moving in straight lines. To avoid confusion, remember that inertia is tied to mass, and Newton first law is often called the law of inertia to highlight this fundamental concept. Final Answer: The tendency of an undisturbed object to remain at rest or keep moving with constant velocity is called inertia.

Question 14

For a simple pendulum, the time taken to complete one full oscillation (one complete to and fro swing) is known as what quantity?

Accepted Answer

Introduction / Context: This question is about simple harmonic motion and pendulums. A simple pendulum swings back and forth about its mean position. One complete oscillation is a full to and fro motion. In describing such periodic motions, physicists use a particular term for the time taken to complete one cycle. Recognising this term is essential for solving problems involving frequency and timing of oscillations. Given Data / Assumptions: The system is a simple pendulum swinging in a vertical plane. One oscillation means going from one extreme position to the other extreme and back again. We are asked for the name of the time taken for one complete oscillation. Options include maximum speed, average speed, time period and time interval. Concept / Approach: In periodic motion, the time taken to complete one full cycle is called the time period, usually denoted by T. For a pendulum, this is the time between successive passages through the same extreme position in the same direction or between successive crossings of the mean position in the same direction. Maximum speed and average speed are different concepts related to velocity, while time interval is a general term for any difference in time, not specifically one cycle. Therefore, time period is the correct answer. Step-by-Step Solution: Step 1: Visualise the pendulum bob moving from the left extreme to the right extreme and back to the left extreme. Step 2: Define one complete oscillation as such a full to and fro motion about the mean position. Step 3: The time taken for this one oscillation is a standard quantity in periodic motion, called the time period T. Step 4: Maximum speed refers to the highest speed reached during motion, which usually occurs at the mean position, not to the duration of a cycle. Step 5: Average speed is the total distance travelled divided by total time taken and is again not specific to one cycle definition. Step 6: Time interval is a generic term for any duration between two events and does not specifically refer to one full oscillation. Step 7: Conclude that the correct term for the time for one complete oscillation is time period. Verification / Alternative check: In formulas for a simple pendulum, such as T = 2 * π * √(L / g), T denotes the time period. Experimental measurements of pendulums involve measuring the time for multiple oscillations and then dividing by the number of oscillations to estimate T. For example, timing 20 swings and dividing the total time by 20 yields the time period. These standard practices and formulas consistently use time period for a single oscillation, confirming the answer. Why Other Options Are Wrong: Maximum speed describes how fast the bob moves at its fastest point and is measured in m/s, not in seconds as time is. Average speed over one or more oscillations is a different quantity and again is not the name for the duration of a single cycle. Time interval is too general and can refer to any duration; it is not the standard technical term in the context of periodic motion. Common Pitfalls: Some students use time interval and time period interchangeably in casual language. In physics, time period has a very specific meaning tied to periodic or oscillatory motion. Always associate the word period with cycles and oscillations. This habit will help you avoid confusion in questions dealing with pendulums, springs, electrical oscillations and waves. Final Answer: The time for one complete oscillation of a pendulum is called its Time period.

Question 15

When the brakes of a moving vehicle are applied and it comes to a stop, which type of force between the tyres and the road surface is primarily responsible for stopping the vehicle?

Accepted Answer

Introduction / Context: This question links everyday experience of braking in vehicles with basic physics of forces. When you press the brake pedal in a car or motorcycle, the vehicle slows down and eventually stops. Many forces act on the vehicle, but one particular contact force between the tyres and the road actually provides the horizontal opposition to motion. Understanding which force is responsible is important for road safety, vehicle design and basic mechanics. Given Data / Assumptions: A vehicle is initially moving along a road. The driver applies the brakes, and the vehicle comes to rest. We want to identify the force that directly causes the vehicle to stop. Options include gravitational force, van der Waals force, frictional force and covalent force. Concept / Approach: When the brakes are applied, brake pads create friction on the wheel or disc, reducing its rotation. The tyres then resist sliding on the road because of friction between rubber and the road surface. This frictional force acts opposite to the direction of motion and provides the net horizontal force that decelerates the vehicle. Gravity acts vertically downward and does not directly oppose the horizontal motion. Van der Waals and covalent forces are microscopic forces at the molecular level and are not used to describe macroscopic braking. Therefore, frictional force is the correct answer. Step-by-Step Solution: Step 1: Identify that applying brakes reduces the rotation of the wheels using friction inside the brake system. Step 2: As the wheels slow down, the tyres try to slide relative to the road. Step 3: Friction between the tyre surface and the road opposes this relative motion and exerts a horizontal force on the tyres. Step 4: According to Newton second law, this horizontal frictional force acts opposite to motion and produces a deceleration that reduces the vehicle speed. Step 5: Over time, the vehicle speed drops to zero and it comes to rest. Step 6: Gravitational force acts vertically downward and is balanced by the normal reaction from the road; these vertical forces do not directly change the horizontal speed. Step 7: Conclude that frictional force between tyre and road is the main force responsible for stopping a vehicle when brakes are applied. Verification / Alternative check: If the road is icy or very slippery, the friction between tyres and road is greatly reduced. In such conditions, even when brakes are applied, the vehicle may skid and take a much longer distance to stop or may not stop in time. This clearly shows that stopping ability depends on friction. On rough dry roads, friction is higher, and braking is more effective. These real world observations confirm that frictional force is the key factor in stopping vehicles. Why Other Options Are Wrong: Gravitational force acts vertically and is always present, but it does not directly oppose the horizontal motion of the vehicle. Van der Waals force is a weak intermolecular force important in chemistry and surface phenomena, not in macroscopic vehicle braking descriptions. Covalent force refers to the strong bonds within molecules, which maintain the integrity of materials but are not used to describe the macroscopic braking process. Common Pitfalls: Some learners think the brake pads themselves stop the vehicle, forgetting that it is the friction between tyres and road that transmits the braking force to the vehicle. If tyres leave the ground or the road is very slippery, braking becomes ineffective even if brake pads are working. Remember that frictional force at the tyre road contact patch is the actual horizontal force that slows down and stops the vehicle. Final Answer: The force that makes a vehicle stop when the brakes are applied is the frictional force between the tyres and the road.

Question 16

Using the definition of electric current, what is the current in amperes when a charge of 500 coulomb flows uniformly through a conductor in 25 seconds?

Accepted Answer

Introduction / Context: This question is a straightforward numerical application of the definition of electric current. Current represents the rate of flow of electric charge through a conductor. If you know how much charge passes through a cross section in a certain time, you can calculate the current easily. Such calculations are common in basic electricity and electronics problems. Given Data / Assumptions: Total charge flowing, Q = 500 coulomb. Time of flow, t = 25 seconds. Charge is assumed to flow uniformly during this interval. We are asked to find the electric current I in ampere. Concept / Approach: Electric current I is defined as the rate of flow of charge, given by the formula I = Q / t, where Q is the total charge in coulomb and t is the time in second. One ampere corresponds to one coulomb of charge passing through a cross section in one second. Substituting the given values into the formula allows a simple division to find the current. Care must be taken to divide correctly and to interpret the result as an ampere value, not a raw number without units. Step-by-Step Solution: Step 1: Write the formula for current: I = Q / t. Step 2: Substitute the given values: Q = 500 C and t = 25 s. Step 3: Compute I = 500 / 25. Step 4: Perform the division: 25 goes into 50 two times, leaving 0, so 500 / 25 = 20. Step 5: Attach the correct unit: I = 20 ampere (A). Step 6: Compare with the options and see that 20 is listed as option B. Step 7: Conclude that the current is 20 A. Verification / Alternative check: As a quick mental check, note that 25 seconds is one quarter of 100 seconds. If 500 coulomb passes in 25 seconds, then in 100 seconds at the same rate, 2000 coulomb would pass. The current would then be 2000 C / 100 s = 20 A, which matches the answer. Another way to see it is that 500 / 25 equals 50 / 2.5, which is 20. These quick checks confirm that the arithmetic is correct and that 20 A is reasonable for many circuits. Why Other Options Are Wrong: 12500 ampere would correspond to 500 C flowing in 0.04 s, which is much larger than the given time and clearly not correct for the data. 40 ampere would imply 500 C in 12.5 s, not in 25 s, so it does not satisfy the given conditions. 6250 ampere would imply 500 C in 0.08 s, which again is inconsistent with 25 s. Common Pitfalls: Common mistakes include inverting the formula to t / Q instead of Q / t or misplacing decimal points when dividing. Another error is forgetting to include units in the answer. Always start by writing I = Q / t, then substitute values carefully and carry the unit ampere through to the final result. Double checking the division mentally can help avoid simple calculation errors in an exam setting. Final Answer: The current is 20 ampere when 500 coulomb of charge flows in 25 seconds.

Question 17

In meteorology, which physical quantity is specifically defined as a measure of the moisture content or water vapour present in air?

Accepted Answer

Introduction / Context: This question again relates to atmospheric science and focuses on the concept used to describe how moist the air is. People often say that the weather feels humid or dry. Scientists and weather reports use a specific term to quantify this moisture content. Identifying that term is important for understanding weather forecasts and comfort levels in different climates. Given Data / Assumptions: We are dealing with the air in the atmosphere. The physical property of interest is the moisture content or water vapour present in the air. Options include density, viscosity, impurity and humidity. We assume standard meteorological definitions. Concept / Approach: Humidity is the term used in meteorology to describe the amount of water vapour in the air. It can be expressed as absolute humidity, relative humidity or specific humidity, but the general concept always refers to moisture content. Density is mass per unit volume of air, viscosity is a measure of internal friction in fluids and impurity is not a standard physical quantity in this context. Therefore, humidity is the correct answer. Step-by-Step Solution: Step 1: Recognise that moisture content in air is associated with water vapour, the gaseous form of water. Step 2: Identify that meteorologists use the word humidity to describe how much water vapour is present. Step 3: Recall that relative humidity is commonly reported in weather reports as a percentage, showing how close the air is to saturation. Step 4: Understand that density is mass per unit volume and can describe air in general but does not specifically refer to moisture content. Step 5: Note that viscosity describes a fluid resistance to flow and is not directly a measure of water vapour content. Step 6: Realise that impurity is a generic idea of contamination and is not a precisely defined thermodynamic quantity for moisture. Step 7: Conclude that humidity is the correct quantity measuring moisture in air. Verification / Alternative check: Weather apps and forecasts regularly include relative humidity values along with temperature and pressure. Devices that measure moisture in air are called hygrometers, and they output humidity readings. Air conditioning systems often control humidity to maintain comfort indoors. All these examples confirm that humidity is indeed the standard term that quantifies water vapour content in air, distinguishing it clearly from density and viscosity. Why Other Options Are Wrong: Density is the mass per unit volume of air or any substance and does not specifically describe the water vapour fraction. Viscosity measures the internal friction or resistance to flow in fluids and does not give a direct measure of moisture content. Impurity is a vague term for contamination and is not a standard physical quantity used to describe moisture in atmospheric science. Common Pitfalls: Students sometimes confuse humidity with density, thinking that moist air must always be denser, but humidity itself is a separate concept. Another source of confusion is mixing up humidity with precipitation. Precipitation involves liquid or solid water falling from clouds, while humidity refers to invisible water vapour in the air. Remember that devices called hygrometers measure humidity, reinforcing the connection between the term humidity and moisture in air. Final Answer: The physical quantity that measures the moisture present in air is Humidity.

Question 18

A particle is thrown vertically upward. When it reaches the highest point of its motion and is instantaneously at rest, what is its acceleration at that point?

Accepted Answer

Introduction / Context: This question tests understanding of motion under gravity, specifically what happens at the highest point when an object is thrown vertically upward. Many learners know that the velocity becomes zero at the top, but they sometimes misunderstand the behaviour of acceleration at that instant. Clarifying this point is very important for correctly solving vertical motion problems in kinematics. Given Data / Assumptions: A particle is thrown straight up in the vertical direction. We consider motion in a uniform gravitational field near the Earth. Air resistance is neglected. We are interested in the state of acceleration at the highest point. Concept / Approach: Under uniform gravity near the Earth surface, the acceleration due to gravity g acts downward at all points along the path, irrespective of the direction of motion. When the particle reaches the highest point, its instantaneous velocity becomes zero, but gravity is still acting. Therefore, the acceleration remains g downward. The fact that velocity is zero at that instant does not mean acceleration is zero. The presence of downward acceleration at the top causes the particle to start moving downward again. Step-by-Step Solution: Step 1: Recognise that throughout the motion, the only significant force on the particle is its weight, mg, acting downward. Step 2: This constant weight produces a constant acceleration g downward, regardless of where the particle is on its path. Step 3: At the highest point, the vertical component of velocity becomes zero, but the force mg is still present. Step 4: Since acceleration is net force divided by mass, and mg is non zero, the acceleration cannot be zero. Step 5: The direction of this acceleration is downward, towards the Earth, with magnitude approximately 9.8 m/s^2. Step 6: Therefore, the correct description is that the particle has a downward acceleration at the highest point. Step 7: As a result of this downward acceleration, the particle begins to move downward immediately after reaching the highest point. Verification / Alternative check: Kinematic equations such as v^2 = u^2 + 2 * a * s can be applied. From the start to the highest point, the acceleration a is constant and equal to g downward (negative if upward is taken as positive). At the top, v = 0, but we use the same value of a as in the rest of the motion. If acceleration were zero at the top, the particle would stay there forever once it stopped, which does not happen in reality. Instead, it immediately falls back, proving that acceleration remains downward g at the highest point. Why Other Options Are Wrong: An upward acceleration would require a net upward force at the highest point, which does not exist in simple free fall under gravity. A downward velocity is incorrect at the highest point, because the velocity at that instant is zero just before changing direction. A horizontal velocity is not part of the described purely vertical motion, so it does not apply here. Common Pitfalls: A frequent misconception is to think that whenever velocity is zero, acceleration must also be zero. This is not true in general. Velocity can be zero at an instant while acceleration is non zero; for example, at turning points in vertical motion or at the extremes of simple harmonic motion. To avoid this mistake, always think in terms of forces. If a net force acts, acceleration is present even if instantaneous velocity is zero. In vertical throws, gravity always acts downward, so acceleration is always downward g. Final Answer: At the highest point of its motion, the particle has a downward acceleration equal to g due to gravity.

Question 19

For a stringed musical instrument, the quality or tone (timbre) of the musical sound it produces depends primarily on which characteristic of the sound wave?

Accepted Answer

Introduction / Context: This question belongs to the physics of sound and music. Musical instruments can produce sounds with the same pitch and loudness but with different qualities or tones. The quality or tone, also called timbre, is what allows you to distinguish between a note played on a guitar and the same note played on a violin. Understanding which physical property of the sound wave controls this quality is important for both physics and music appreciation. Given Data / Assumptions: The instrument is a stringed musical instrument, such as a guitar, violin or sitar. The property of sound considered is quality or tone (timbre), not loudness or pitch. Options mention frequency, string length, amplitude and waveform. We assume standard definitions of pitch, loudness and quality in acoustics. Concept / Approach: In sound, three basic characteristics are often distinguished: pitch, loudness and quality. Pitch is mainly determined by the frequency of vibration. Loudness is related to intensity and thus to amplitude. Quality or tone depends on the waveform of the sound, which includes the mixture of fundamental and harmonic frequencies present. Different instruments produce different combinations of harmonics, leading to distinct waveforms even for the same fundamental frequency. Therefore, quality depends primarily on the waveform of the sound, not simply on frequency or amplitude alone. Step-by-Step Solution: Step 1: Recall that pitch corresponds to the perceived highness or lowness of a sound and is mainly controlled by the frequency of vibration. Step 2: Recognise that loudness is associated with the energy and intensity of the sound, which depends on the amplitude of vibration. Step 3: Understand that quality or tone is what distinguishes different sources producing the same pitch and loudness, such as different musical instruments. Step 4: Realise that different instruments generate different sets of overtones and harmonics in addition to the fundamental frequency. Step 5: These harmonics combine to create a unique resultant waveform for each instrument. Step 6: Since the waveform shape encodes the mixture of harmonics, it is this waveform that determines the quality or tone. Step 7: Therefore, among the options, waveform of the sound is the correct factor for quality. Verification / Alternative check: If you analyse sounds from a guitar and a flute playing the same note on an oscilloscope, the frequencies and overall loudness can be adjusted to be similar, but the waveform shapes remain distinctly different. Fourier analysis shows that the harmonic content of each instrument is different. This change in waveform, not the fundamental frequency alone, is what gives each instrument its unique timbre. These observations agree with the idea that quality depends on the waveform of the sound. Why Other Options Are Wrong: Frequency of vibration primarily controls pitch, not the quality or tone, although it indirectly influences which harmonics are possible. Length of the strings affects the fundamental frequency and thus pitch, but tone quality is determined by the full harmonic structure and waveform. Amplitude determines loudness, not quality; a sound can be louder or softer without changing its characteristic tone. Common Pitfalls: Students sometimes confuse pitch and quality because both depend on vibration properties. It is crucial to separate roles: frequency for pitch, amplitude for loudness and waveform for quality. The length, tension and thickness of strings affect frequency and possible harmonics but the immediate physical quantity representing quality in wave language is the waveform. Keeping this mapping clear helps you quickly answer conceptual questions about sound characteristics. Final Answer: The quality or tone of a musical sound from a stringed instrument depends chiefly on the waveform of the sound it produces.

Question 20

When any ordinary hot object is heated, which type of electromagnetic radiation does it predominantly emit as thermal energy?

Accepted Answer

Introduction / Context: When an object is heated, it does not simply glow in the visible region; it actually emits energy over a wide range of electromagnetic wavelengths. In basic physics and general science examinations, you are often asked which radiation is most strongly associated with the thermal emission of everyday hot objects such as a hot iron, a cup of tea, or a body with normal body temperature. This question tests your understanding of thermal radiation and the electromagnetic spectrum. Given Data / Assumptions: - We are dealing with ordinary hot objects at temperatures comparable to everyday environments, not stars or extremely hot plasmas.- The object is at a temperature above its surroundings, so it is radiating heat energy.- We want the main or predominant type of radiation associated with the heat that we feel from such objects. Concept / Approach: All objects above absolute zero emit electromagnetic radiation known as thermal radiation. The distribution of wavelengths depends on the temperature. For typical temperatures encountered in daily life, the peak of this emission lies in the infrared region of the electromagnetic spectrum. Infrared radiation is strongly associated with heat; thermal cameras and remote temperature sensors detect infrared rather than visible light. Only at very high temperatures does visible light emission become dominant, such as the white glow of an incandescent filament. Step-by-Step Solution: 1. Recognise that hot objects radiate energy in the form of electromagnetic waves.2. Recall the electromagnetic spectrum: radio, microwave, infrared, visible, ultraviolet, X-rays, and gamma rays.3. Note that everyday hot objects, with temperatures of a few hundred kelvin, have peak emission in the infrared band.4. Understand that the warmth you feel when you are near a hot object is primarily due to infrared radiation reaching your skin.5. Conclude that the correct answer is infrared rays, as they most directly represent the thermal radiation of ordinary hot bodies. Verification / Alternative check: You can verify this concept by considering devices that detect body heat, such as night vision goggles and thermal cameras. These devices are designed to detect infrared radiation, not visible light or X-rays. Also, even when an object is hot but does not visibly glow, you still feel its warmth, which again indicates emission in the infrared region rather than visible light. Why Other Options Are Wrong: - Visible light: Emitted significantly only when the object is extremely hot, such as a filament lamp or molten metal, not for all hot objects.- Ultraviolet rays: These are associated with very high energy and very high temperatures, such as the surface of the sun, not common hot objects.- X-rays: Require extremely high energy transitions in atoms and are never the characteristic emission of ordinary hot materials. Common Pitfalls: Many students mistakenly think of visible light because they associate heat with glowing objects. However, an object can be very hot and still not glow visibly, yet it strongly emits infrared radiation. Another mistake is to assume that more harmful or energetic radiations like X-rays or ultraviolet rays must be involved whenever heat is mentioned, which is not correct for normal temperatures. Final Answer: The correct answer is that every ordinary hot object predominantly emits infrared rays as thermal radiation.

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