Question 1

Compound Pendulum – Frequency of Oscillation
Given a compound pendulum with radius of gyration k_G about its centroidal axis and distance h between the point of suspension and the center of gravity (C.G.), what is the frequency of small oscillation…

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Introduction / Context: A compound (physical) pendulum is any rigid body oscillating about a horizontal axis not passing through its center of mass. Its period and frequency depend on geometry and mass distribution via the radius of gyration and the suspension distance. Given Data / Assumptions: Small angular oscillations (small-angle approximation). Radius of gyration about C.G.: k_G. Distance between suspension point and C.G.: h. Acceleration due to gravity: g. Concept / Approach: The time period T of a compound pendulum is T = 2π * sqrt( (k_G^2 + h^2) / (g * h) ). Frequency is the reciprocal: f = 1 / T. Substituting gives f = (1 / 2π) * sqrt( g * h / (k_G^2 + h^2) ). Step-by-Step Solution: Start with T = 2π * sqrt( (k_G^2 + h^2) / (g * h) ). Invert to get f = 1 / T. Thus f = (1 / 2π) * sqrt( g * h / (k_G^2 + h^2) ). Verification / Alternative check: Special case: if k_G → 0 (point mass at distance h), f reduces to (1 / 2π) * sqrt( g / h ), the simple pendulum result with length L = h. This matches known theory. Why Other Options Are Wrong: Option B swaps numerator and denominator, giving period instead of frequency form. Options C and D ignore k_G and only apply to a simple pendulum. Option E has incorrect dependence on k_G and h. Common Pitfalls: Confusing radius of gyration with geometric length. Forgetting that frequency is 1 / T, not 2π / T. Final Answer: f = (1 / 2π) * sqrt( g * h / (k_G^2 + h^2) )

Question 2

Simple Harmonic Motion – Amplitude vs Reference Circle
In the geometric representation of simple harmonic motion (SHM) as the projection of uniform circular motion, the amplitude is always _____ the radius of the reference circle.

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Introduction / Context: SHM is often visualized by projecting a point moving uniformly on a circle onto a diameter. This geometric interpretation provides direct insight into amplitude, phase, and displacement relations. Given Data / Assumptions: A particle moves in SHM with maximum displacement A from the mean position. The reference circle has radius R. Projection is taken on a fixed diameter. Concept / Approach: The displacement x at any instant equals the projection of the circular motion radius onto the diameter: x = R cos(θ) (or sin(θ) depending on reference). The maximum value of |x| occurs when cos(θ) = ±1, giving |x|max = R. Hence amplitude A equals the radius R. Step-by-Step Solution: Represent SHM by uniform circular motion of radius R. Displacement along the diameter: x = R cos(θ). Maximum |x| is R. Therefore amplitude A = R. Verification / Alternative check: Using standard SHM form x(t) = A cos(ω t + φ). Compare with circular projection x = R cos(θ). Matching forms implies A = R. Why Other Options Are Wrong: “less than” / “greater than” / “twice” / “half”: contradict the direct geometric identity A = R. Common Pitfalls: Confusing amplitude with instantaneous displacement. Final Answer: equal to

Question 3

Moment of a Force – Comprehensive Definitions
Which statement correctly characterizes the moment (torque) of a force about a point?

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Introduction / Context: The moment (or torque) is central to statics and dynamics. Multiple equivalent definitions exist—algebraic, geometric, and vectorial—which are useful in different problem settings. Given Data / Assumptions: A force F acts on a rigid body. A reference point O is specified for taking moments. Planar setting for scalar moment; vector cross product is the general form. Concept / Approach: Scalar magnitude of moment is M_O = F * d, where d is the perpendicular distance from O to the line of action. The direction (sign) depends on rotation sense. Geometrically, the moment magnitude also equals twice the area of the triangle formed by O and the force vector’s line, consistent with vector cross product properties. Step-by-Step Solution: Turning effect: the essence of torque—tendency to rotate. Perpendicular distance form: M_O = F * d (most common in statics). Geometric area form: |r × F| equals 2 × triangle area with sides r and F. All statements express the same physical quantity. Verification / Alternative check: Vector definition: M_O = r × F. Magnitude |M_O| = |r| * |F| * sin(θ). This equals F times the perpendicular distance and also equals twice the triangle area (½ |r||F|sinθ × 2). Why Other Options Are Wrong: “none of the above” is incorrect because all listed statements are valid descriptions of moment. Common Pitfalls: Using non-perpendicular distances in moment calculations. Forgetting sign convention when summing moments for equilibrium. Final Answer: all of the above

Question 4

Linear Momentum – Total Motion Possessed by a Body
In dynamics, the total motion possessed by a body is termed:

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Introduction / Context: Momentum encapsulates both how much matter an object has and how fast it moves, making it a fundamental conserved quantity in isolated systems. This question checks recognition of the standard term for “total motion.” Given Data / Assumptions: A particle of mass m moving with velocity v. Standard SI units: kg for mass, m/s for velocity. Concept / Approach: Linear momentum p is defined by p = m * v. It is a vector quantity pointing in the direction of velocity. In the absence of external force, the total momentum of a system remains constant (principle of conservation of momentum). Step-by-Step Solution: Compute momentum: p = m * v. Interpretation: measures “quantity of motion.” Contrast with mass (amount of matter) and weight (gravitational force m * g). Impulsive force relates to abrupt force action; impulse J = ∫F dt equals change in momentum. Verification / Alternative check: Newton’s second law in impulse form: Δp = J. If no external impulse acts, Δp = 0, so momentum is conserved, reinforcing its role as “total motion.” Why Other Options Are Wrong: mass: property of matter only. weight: force due to gravity, not motion. impulsive force / impulse: interaction and its time integral, not a persistent state variable like momentum. Common Pitfalls: Confusing impulse (N·s) with momentum (also N·s in units) without noting impulse is an action, momentum a state. Final Answer: momentum

Question 5

Force Systems – Definition Check for Coplanar Non-Concurrent Forces
Evaluate the statement: “Forces that lie in the same plane but do not meet at a single point are called coplanar non-concurrent forces.”

Accepted Answer

Introduction / Context: Classifying force systems—concurrent, parallel, coplanar, spatial—guides which equilibrium equations and reduction methods to use. This item checks the precise wording of “coplanar non-concurrent forces.” Given Data / Assumptions: A set of two or more forces acting on a rigid body. All lines of action lie within a single geometric plane. The lines of action do not intersect at one common point. Concept / Approach: Terminology: Coplanar: lines of action in the same plane. Concurrent: lines of action meeting at a single point. Non-concurrent: no single intersection point common to all forces. Therefore, “coplanar non-concurrent forces” is the correct phrase for forces in a plane that are not all meeting at a common point. Step-by-Step Solution: Check coplanarity: yes—same plane. Check concurrency: no—no single common intersection point. Therefore, the given statement matches the standard definition. Verification / Alternative check: Examples include general 2D force systems on a rigid body with forces at different points and directions. Equilibrium requires ΣF_x = 0, ΣF_y = 0, and ΣM_O = 0 because moments cannot be reduced by concurrency. Why Other Options Are Wrong: “Disagree”: contradicts standard definitions. “Agree only if they are parallel”: parallel forces are a subset; non-concurrency does not require parallelism. “Agree only if they are concurrent”: self-contradictory. “Agree only for two forces”: applies to any number of forces. Common Pitfalls: Confusing “coplanar non-concurrent” with “parallel” or “concurrent”. Final Answer: Agree

Question 6

Statics — Equilibrium Condition for a Rigid Body If the resultant (net force) of a number of forces acting on a body is zero, does it automatically imply that the body is not in equilibrium?

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Introduction / Context: In engineering mechanics, equilibrium for a rigid body requires that both the net force and the net moment about any point are zero. The prompt claims that a zero resultant force means the body will not be in equilibrium, which tests understanding of complete equilibrium conditions. Given Data / Assumptions: A body (rigid body perspective, not just a particle). Multiple forces act on the body; their vector sum is zero. No other information about moments is given. Concept / Approach: For a particle, equilibrium is achieved if the net force is zero. For a rigid body, two independent requirements must be met: ΣF = 0 and ΣM = 0. If only ΣF = 0 is satisfied, couple moments may still cause rotation, preventing equilibrium. Step-by-Step Solution: Check force balance: resultant force = 0 (given). Check moment balance: not provided; a pure couple could exist so ΣM ≠ 0. Therefore, zero resultant force alone does not imply “not in equilibrium” nor does it guarantee equilibrium; the statement in the stem is false. Verification / Alternative check: Consider two equal, opposite, and parallel forces forming a couple. Net force is zero but there is a net moment. The body rotates, so it is not in equilibrium despite zero resultant force. Why Other Options Are Wrong: “Yes” contradicts rigid-body equilibrium theory; “It depends on the reference frame only” ignores the essential role of moments; “Indeterminate…” is partly true diagnostically, but the question asks whether the statement is correct—hence “No”. Common Pitfalls: Confusing particle equilibrium (ΣF = 0) with rigid-body equilibrium (ΣF = 0 and ΣM = 0); forgetting that couples do not change ΣF but do change ΣM. Final Answer: No, zero resultant does not by itself rule out equilibrium.

Question 7

Rotational Dynamics — Definition Clarification What is the most precise definition of the mass moment of inertia (as used in dynamics)?

Accepted Answer

Introduction / Context: The phrase “moment of inertia” is used in two nearby but distinct contexts: mass moment of inertia in dynamics and area moment of inertia in strength of materials. This question checks the exact definition relevant to rotational dynamics of bodies. Given Data / Assumptions: We are talking about mass and rotation of rigid bodies (dynamics). Standard SI usage and definitions. Concept / Approach: The mass moment of inertia I about an axis is defined by integrating the squared distance of each mass element to that axis: I = ∫ r^2 dm. Because of the r^2 term, it is the second moment of the mass distribution. Step-by-Step Solution: Recognize context: dynamics ⇒ mass distribution relative to an axis. Formal definition: I = ∫ r^2 dm, which is a second moment of mass. Therefore, the correct choice is “second moment of mass”. Verification / Alternative check: Contrast with strength of materials: area moment of inertia (second moment of area) I_z = ∫ y^2 dA is used in bending/deflection, not in rotational dynamics equations like T = I * α. Why Other Options Are Wrong: “Second moment of force” is not a standard concept; “Second moment of area” describes section properties, not kinetic rotational inertia; “All of these” mixes distinct quantities. Common Pitfalls: Using area properties in place of mass properties when computing kinetic energy, angular acceleration, or gyroscopic effects. Final Answer: Second moment of mass.

Question 8

Area Moments — Hollow Circular Section For a hollow circular section, the polar moment of inertia about an axis perpendicular to the section (through its centroid) is how many times the centroidal moment of inertia about the X–X axis (in-plane princ…

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Introduction / Context: For axisymmetric circular sections (solid or hollow), in-plane centroidal moments about X–X and Y–Y are equal. The polar moment about the perpendicular centroidal axis equals the sum of these two equal in-plane moments. Given Data / Assumptions: Hollow circular section with centroid at the geometric center. Principal in-plane axes X–X and Y–Y through the centroid. Polar axis Z perpendicular to the section through the centroid. Concept / Approach: By definition, J_z (polar) = I_x + I_y. For any circular (axisymmetric) section, I_x = I_y. Hence J_z = 2 I_x. Therefore, the polar moment is two times either in-plane centroidal moment. Step-by-Step Solution: Symmetry: I_x = I_y for a circular annulus. Polar relation: J_z = I_x + I_y. Thus J_z = 2 I_x ⇒ “Two times”. Verification / Alternative check: For a thin ring of radius R and area A, I_x = I_y = (A R^2)/2 and J_z = A R^2. Indeed, J_z = 2 * I_x holds exactly. Why Other Options Are Wrong: “Same” ignores J_z = I_x + I_y; “Half” and “Four times” contradict the exact identity for axisymmetric sections. Common Pitfalls: Confusing mass polar moment with area polar moment; misapplying the relation J = I_x + I_y to non-centroidal axes. Final Answer: Two times.

Question 9

Theory of Machines — Law of the Machine For a lifting machine, the linear relationship between effort P and load W is called the law of the machine. Which form is correct (m is the slope constant and C is the constant representing friction)?

Accepted Answer

Introduction / Context: Real machines require extra effort beyond the ideal proportional term due to friction and other losses. This relationship is captured by the law of the machine, which is a straight line when P is plotted against W. Given Data / Assumptions: P: effort applied; W: load lifted. m: proportionality constant (slope); C: constant effort to overcome friction and other losses. Concept / Approach: Experimentally, P varies linearly with W. The y-intercept at W = 0 equals the constant C (pure friction effort), and slope m represents how much extra effort is needed per unit load. Step-by-Step Solution: At W = 0, P must equal friction effort ⇒ P = C (positive). Therefore, the correct linear form is P = mW + C, a straight line with positive intercept and slope. Verification / Alternative check: Plotting test data (P vs W) for hoists or jacks typically yields a line with intercept C > 0, confirming the “+ C” term. Why Other Options Are Wrong: “P = mW − C” would give negative effort at small loads; “m/W” is not linear in W; “C − mW” implies decreasing effort with increasing load, which is unphysical for lifting machines. Common Pitfalls: Confusing ideal mechanical advantage (no friction) with the real line including C; misreading axes (P vs W) when interpreting experimental graphs. Final Answer: P = mW + C.

Question 10

Rigid Body Dynamics — Solid Sphere What is the mass moment of inertia of a solid sphere of mass m and radius r about any diameter?

Accepted Answer

Introduction / Context: The mass moment of inertia of standard 3D bodies is fundamental in dynamics and vibration. The solid sphere’s inertia about a diameter is a classic closed-form result used in rolling motion and rotational kinetic energy calculations. Given Data / Assumptions: Uniform solid sphere of mass m, radius r. Axis: any diameter through the center (symmetry ensures all diameters are equivalent). Concept / Approach: For a sphere, I_diameter is obtained by integrating r_perp^2 over the full volume with constant density. The standard result is I = 2/5 m r^2. Step-by-Step Solution (outline): Use spherical coordinates and exploit symmetry to evaluate ∫ r_perp^2 dm. The integral yields I = (2/5) m r^2 for a uniform solid sphere. Verification / Alternative check: Compare with a thin spherical shell (hollow sphere): I_shell = (2/3) m r^2, which is larger because more mass is located farther from the center. The solid sphere must have a smaller coefficient than 2/3, and 2/5 satisfies this. Why Other Options Are Wrong: “2 m r^2 / 3” is for a thin spherical shell, not a solid sphere; “m r^2” and “m r^2 / 2” are not correct for any standard spherical distribution about a diameter. Common Pitfalls: Mixing up solid vs hollow results; confusing diameter-axis inertia with polar moments of other shapes. Final Answer: 2 m r^2 / 5.

Question 11

Kinetics on an Incline — Component of Weight For a smooth (frictionless) inclined plane at angle θ to the horizontal, what is the linear acceleration of a body sliding down the plane?

Accepted Answer

Introduction / Context: Motion on an incline is one of the first applications of Newton’s second law. Decomposing weight into components parallel and perpendicular to the plane gives the acceleration for a frictionless slide. Given Data / Assumptions: Incline angle θ. Frictionless contact (smooth plane). Neglect air resistance; body treated as a particle. Concept / Approach: Resolve weight mg into a component parallel to the plane mg sin θ (down the slope) and a normal component mg cos θ. With no friction, the only unbalanced force along the plane is mg sin θ, producing acceleration a. Step-by-Step Solution: ΣF_parallel = m a ⇒ m a = m g sin θ. Therefore a = g sin θ. Verification / Alternative check: Boundary checks: θ = 0° ⇒ a = 0 (horizontal surface, no sliding force). θ = 90° ⇒ a = g (free fall vertically). The expression a = g sin θ satisfies both limits. Why Other Options Are Wrong: “g cos θ” is the normal reaction divided by m; “g tan θ” does not represent net acceleration; “Zero” ignores the nonzero parallel component for θ > 0. Common Pitfalls: Mixing up sine and cosine with the chosen angle reference; forgetting to remove friction only from the parallel direction while keeping the normal reaction. Final Answer: g sin θ.

Question 12

Friction — Type at Rest The friction experienced by a body when it is at rest (before impending motion) is called:

Accepted Answer

Introduction / Context: Frictional behavior differs for bodies at rest versus in motion. Correct terminology matters when calculating forces needed to initiate versus maintain motion in machine elements and contact problems. Given Data / Assumptions: Body is at rest relative to the surface. External force may be applied but remains below the threshold that causes slipping. Concept / Approach: Static friction adjusts up to a maximum value to oppose impending motion: F_s ≤ μ_s N. The maximum value at the verge of sliding is called limiting friction (F_lim = μ_s N). Once motion starts, the resisting force becomes kinetic/dynamic friction F_k = μ_k N with μ_k < μ_s typically. Step-by-Step Solution: Identify motion state: at rest ⇒ static regime. Force response: friction equals whatever is required (up to F_lim) to maintain rest. Terminology: the general friction when at rest is “static friction”; its peak value is “limiting friction”. Verification / Alternative check: Simple block-on-plane experiment: gradually increase pull; friction increases correspondingly until a maximum is reached (limiting), after which motion begins and friction drops to kinetic. Why Other Options Are Wrong: “Dynamic friction” refers to motion; “Limiting friction” is a specific maximum value, not the general name; “Coefficient of friction” is a dimensionless property μ, not a force. Common Pitfalls: Using μ_s N (limiting) for all static cases; forgetting that static friction can be less than its maximum depending on applied load. Final Answer: Static friction.

Question 13

Area Moments — Triangle About Different Parallel Axes For a triangular section of base b and height h, the moment of inertia about an axis through the vertex and parallel to the base is how many times the moment of inertia about the centroidal axis …

Accepted Answer

Introduction / Context: Parallel-axis transformations are common in structural analysis. For triangles, knowing the relationship between inertia about centroidal and non-centroidal axes helps in composite-section calculations. Given Data / Assumptions: Triangle with base b and height h. Centroidal axis parallel to base with known inertia I_G = (b h^3) / 36. Vertex axis: a line through the vertex, parallel to the base. Concept / Approach: Use the parallel-axis theorem: I_vertex = I_G + A d^2, where A is the area and d is the perpendicular distance between the two parallel axes. For a triangle, the centroid lies at h/3 from the base and at 2h/3 from the opposite vertex. Step-by-Step Solution: Area: A = (1/2) b h. Centroid-to-vertex distance: d = 2h / 3. Centroidal inertia: I_G = (b h^3) / 36. Apply parallel-axis: I_vertex = I_G + A d^2 = (b h^3)/36 + (1/2 b h) * (2h/3)^2. Compute increment: (1/2 b h) * (4h^2/9) = (2/9) b h^3 = (8/36) b h^3. Sum: I_vertex = (1/36 + 8/36) b h^3 = (9/36) b h^3 = (1/4) b h^3. Ratio: I_vertex / I_G = (1/4) / (1/36) = 9. Verification / Alternative check: Numeric example with b = h = 1 yields I_G = 1/36 and I_vertex = 1/4; the ratio is indeed 9. Why Other Options Are Wrong: Two, four, and six times do not satisfy the exact parallel-axis calculation for a triangle. Common Pitfalls: Using h/3 instead of 2h/3 for the centroid-to-vertex distance; mixing up which axis is being referenced. Final Answer: Nine times.

Question 14

Statics — Force System Classification Coplanar non-concurrent forces are those forces which ______ at one point, while their lines of action lie in the same plane.

Accepted Answer

Introduction / Context: Classifying systems of forces guides the choice of equilibrium equations and solution strategy. “Coplanar” refers to forces whose lines of action lie in a single plane; “concurrent” refers to whether those lines intersect at a common point. Given Data / Assumptions: All forces are coplanar (2D). Their lines of action do not intersect at one point. Concept / Approach: Coplanar concurrent forces meet at a single point. Coplanar non-concurrent forces do not all intersect at one point; they may be parallel, form a general 2D system, or intersect pairwise at different points. Such systems typically require both ΣF = 0 components and ΣM = 0 to solve. Step-by-Step Reasoning: Check plane: forces are coplanar ⇒ 2D statics applies. Check concurrency: non-concurrent ⇒ no single common intersection. Definition therefore: they “do not meet” at one point. Verification / Alternative check: Examples include a beam with multiple non-collinear loads and reactions. Not all lines of action intersect at one point, yet all lie in the beam’s plane. Why Other Options Are Wrong: “Meet” defines concurrent forces; “Must be parallel only” is too restrictive (non-concurrent sets can be non-parallel); “Equal and opposite” would be a special case forming a couple. Common Pitfalls: Confusing “non-concurrent” with “parallel”; assuming coplanar automatically implies concurrency. Final Answer: do not meet.

Question 15

Simple machine — wheel and axle (velocity ratio)

For a simple wheel and axle, if D is the diameter of the effort wheel and d is the diameter of the load axle, what is the velocity ratio (V.R.)?

Accepted Answer

Introduction / Context: The wheel and axle is a classic simple machine used to amplify an applied effort. Understanding its velocity ratio (V.R.) helps in computing mechanical advantage and efficiency, which are vital in lifting mechanisms and hoisting equipment. Given Data / Assumptions: A rigid effort wheel of diameter D is keyed to a rigid axle of diameter d. There is no slip between rope and wheel/axle (pure rolling of the rope on the rim). Small motions are considered so linear travel equals the arc length at the rims. Concept / Approach: Velocity ratio is defined as V.R. = distance moved by effort / distance moved by load (for the same time interval). In one full revolution, the effort point on the wheel rim travels a circumference πD, while the load point on the axle rim travels πd. Hence V.R. = πD / πd = D / d. Step-by-Step Solution: Define V.R. = effort distance / load distance.For one revolution: effort distance = πD.For one revolution: load distance = πd.Therefore V.R. = (πD) / (πd) = D / d. Verification / Alternative check: If D = d, V.R. = 1, meaning no amplification of motion; this matches intuition. If D is much larger than d, V.R. increases proportionally, indicating greater motion advantage. Why Other Options Are Wrong: D + d and D − d: These do not arise from circumference ratios. D × d: Multiplies lengths, giving units of area, not a dimensionless ratio. d / D: This would be the inverse; it decreases as D grows, contradicting physical behavior. Common Pitfalls: Confusing mechanical advantage with velocity ratio. For an ideal machine, M.A. = V.R., but V.R. is defined purely by geometry and motion, independent of friction. Final Answer: D / d

Question 16

Unit conversion — how many newtons are in one kilogram-force (kgf)?

Accepted Answer

Introduction / Context: Engineering problems often require converting between kilogram-force (kgf) and newtons (N). The kilogram-force is a gravitational unit of force, while the newton is the SI unit. Knowing the approximate conversion is essential for quick calculations. Given Data / Assumptions: Standard gravitational acceleration g ≈ 9.8 m/s^2 (more precisely 9.80665 m/s^2). 1 kilogram-force is defined as the weight of a 1 kg mass under standard gravity. Concept / Approach: Weight W equals mass m times gravitational acceleration g. Thus, the force corresponding to 1 kgf is W = 1 kg * g newtons. Using the standard value of g, this gives approximately 9.8 newtons. Step-by-Step Solution: Start with W = m * g.Set m = 1 kg and g ≈ 9.8 m/s^2.Therefore, 1 kgf ≈ 9.8 N.Using the conventional standard, 1 kgf = 9.80665 N; rounded to one decimal place, 9.8 N. Verification / Alternative check: Compare with everyday approximations: many handbooks use 10 N as a quick mental estimate for 1 kgf; the more precise rounded value for exams is 9.8 N. Why Other Options Are Wrong: 7.8 N and 8.9 N: Underestimate the gravitational force for 1 kg. 12 N: Overestimates standard gravity. 10.8 N: Not a standard approximation; too large. Common Pitfalls: Confusing mass (kg) and force (N). Mass is an intrinsic property; force depends on g. Always multiply mass by g to obtain weight in newtons. Final Answer: 9.8 N

Question 17

To fully determine the effect of a force acting on a rigid body, which properties must be known?

Accepted Answer

Introduction / Context: When analyzing forces in statics and dynamics, it is not enough to know how big a force is. The full effect on a rigid body depends on where and in what direction the force acts. This question tests understanding of the complete specification of a force system. Given Data / Assumptions: Rigid body assumptions (no deformation). Force represented as a vector applied along a line of action. Sense of action (push or pull) matters for boundary conditions and contact behavior. Concept / Approach: A force applied to a rigid body is fully described by: (1) magnitude, (2) direction and line of action (including point or line of application), and (3) sense (push vs. pull). Changing the line of action, even with the same magnitude and direction, can change the induced moment and thus the body’s response. Step-by-Step Solution: Identify components: magnitude determines the strength of action.Direction and specific line of action determine the moment about points/axes.Sense (push/pull) clarifies orientation with respect to supports or members.Hence, all listed properties are required for complete determination. Verification / Alternative check: Consider two equal forces of 100 N parallel to each other but displaced by a distance: together they can form a couple even if magnitudes are equal, proving line of action matters. Why Other Options Are Wrong: Only magnitude: Insufficient because moments depend on position and direction. Only line of action: Without magnitude, the effect cannot be quantified. Only nature (push/pull): Does not determine size or rotational effects alone. Common Pitfalls: Ignoring point of application in beam problems; treating all forces of the same magnitude as equivalent regardless of their lines of action. Final Answer: All of the above

Question 18

Resultant of forces meeting at a point — compute magnitude from components

If several forces act at a point, the magnitude of the single resultant R in terms of the algebraic sums of horizontal and vertical components (ΣH and ΣV) is:

Accepted Answer

Introduction / Context: Vector addition of forces is fundamental. When multiple forces act at a point, we commonly resolve them into orthogonal components and then recombine to find a single equivalent resultant force. Given Data / Assumptions: All forces are concurrent (meet at a single point). Components are resolved along mutually perpendicular axes (horizontal H and vertical V). ΣH and ΣV denote algebraic sums of components. Concept / Approach: The resultant of orthogonal components follows the Pythagorean relationship. If R has components (ΣH, ΣV), then the magnitude of R is given by R = sqrt( (ΣH)^2 + (ΣV)^2 ). The direction θ satisfies tan(θ) = ΣV / ΣH. Step-by-Step Solution: Resolve each force F_i into H and V components.Add components: ΣH = ΣF_ix and ΣV = ΣF_iy.Compute magnitude: R = √( (ΣH)^2 + (ΣV)^2 ).Optionally find direction: θ = atan2(ΣV, ΣH). Verification / Alternative check: Check special case: If ΣH = 0, then R = |ΣV|; if ΣV = 0, then R = |ΣH|. This is consistent with the square root expression. Why Other Options Are Wrong: (ΣH)^2 + (ΣV)^2 without a square root: Gives the squared magnitude, not the magnitude. (ΣH + ΣV)^2 or ΣH + ΣV: Mixing components algebraically destroys vector nature. √(ΣH + ΣV): Nondimensional and incorrect. Common Pitfalls: Forgetting to take the square root; sign errors when summing components; incorrect angle quadrant due to signs of ΣH and ΣV. Final Answer: R = √( (ΣH)^2 + (ΣV)^2 )

Question 19

Ideal machine check — work and efficiency

A lifting machine raises a 1000 N load through 0.2 m using an effort of 200 N through 1.0 m. Is this machine ideal (100% efficient)?

Accepted Answer

Introduction / Context: An ideal machine has no losses, so input work equals output work and efficiency equals 100%. This question tests your ability to verify ideality by comparing work in and work out. Given Data / Assumptions: Load = 1000 N; load distance = 0.2 m. Effort = 200 N; effort distance = 1.0 m. No additional losses specified (we compute purely from given values). Concept / Approach: Efficiency η = output work / input work. If η = 1 (or 100%), the machine is ideal. Also, for an ideal machine, Mechanical Advantage (M.A.) = Velocity Ratio (V.R.). Step-by-Step Solution: Compute output work: W_out = Load * load distance = 1000 * 0.2 = 200 J.Compute input work: W_in = Effort * effort distance = 200 * 1.0 = 200 J.Efficiency: η = W_out / W_in = 200 / 200 = 1 (100%).Check M.A. and V.R.: M.A. = Load / Effort = 1000 / 200 = 5; V.R. = effort distance / load distance = 1.0 / 0.2 = 5; hence M.A. = V.R., consistent with an ideal machine. Verification / Alternative check: Because work in equals work out exactly, there are no implied energy losses. This confirms the ideal assumption from the given numbers. Why Other Options Are Wrong: Incorrect: Conflicts with equal input and output work. Correct only if friction is ignored: Already implied by equal works; the data shows no loss. Correct only if V.R. = 10: Irrelevant; V.R. here is 5. Incorrect because M.A. ≠ V.R.: They are equal in this case. Common Pitfalls: Mixing up force ratios with work equality; forgetting that efficiency compares energies, not just forces or distances independently. Final Answer: Correct

Question 20

Friction at contacts — ladder on rough ground and smooth wall

A ladder rests on rough ground and leans against a perfectly smooth vertical wall. How does the frictional force act at the upper contact with the wall?

Accepted Answer

Introduction / Context: Contact friction depends on the nature of the surfaces. A smooth wall implies no tangential resistance, which is a standard assumption in ladder equilibrium problems in engineering statics. Given Data / Assumptions: Ground is rough (can provide friction). Wall is perfectly smooth (frictionless). Ladder is in static equilibrium. Concept / Approach: At a smooth contact, only the normal reaction can act; there is no frictional component along the surface. Therefore, at the upper end (ladder–wall contact), friction is zero and the reaction is purely normal, i.e., horizontal, directed from the wall to the ladder. Step-by-Step Solution: Identify forces: weight acts at the ladder’s centre of mass; ground provides normal reaction and friction; wall provides only normal reaction (no friction).Because the wall is smooth, tangential (vertical) friction at the wall is zero.Equilibrium equations confirm ground friction balances the tendency to slide.Therefore, friction at the upper end is zero. Verification / Alternative check: Consider a limiting case: if both wall and ground were smooth, the ladder could not be in equilibrium unless special geometry or external support existed, highlighting the role of ground friction alone in the given setup. Why Other Options Are Wrong: Downward/Upward at the upper end: These imply nonzero wall friction, contradicting the smooth-wall assumption. Perpendicular to the wall at the upper end: That describes the wall’s normal reaction, not friction. Horizontal toward the wall: Again, this is the normal reaction direction, not friction. Common Pitfalls: Confusing normal reaction with friction; forgetting that a smooth surface cannot transmit tangential force. Final Answer: Zero at the upper end

Engineering Mechanics Questions