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

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.