eNotes on Work, Energy and Power
In Physics, work, energy, and power have precise meanings. Work is done when a force moves an object in the direction of that force. For example, if you push a box and it moves, you do work. But if you push a wall that doesn’t move, no work is done. Work can be positive (force and motion in the same direction) or negative (like friction opposing motion). Its unit is the Joule (J). When force is not constant, we use the area under the force–displacement graph or integration to calculate work. Simple examples help us understand these concepts clearly.
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The content of these eNotes are briefly explained below:
The concept of Work, Energy, and Power in Physics often differs from our everyday understanding. For example, a porter carrying a load on his head may seem to be doing work, but in Physics, no work is done because the force applied is vertical while displacement is horizontal. Similarly, a person pushing a wall exerts force but does no work if the wall does not move. Physics defines work as the product of force and displacement in the direction of that force. Mathematically, when a force F is applied at an angle θ to displacement S, only the component F cos θ contributes, so work done is W = (F cos θ) S. Work is a scalar quantity.
There are situations where no work is done: when force is zero, when displacement is zero, or when the force is perpendicular to displacement. The unit of work in SI is the Newton-meter or Joule (J), while in the cgs system it is the erg.
Work can be positive or negative. It is positive if force and displacement are in the same direction (e.g., gravity acting on a falling body) and negative if they are in opposite directions (e.g., friction opposing motion).
Illustrative examples explain applications:
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A 100 N force pulling a box at 60° over 8 m does 400 J of work.
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A 10 kg carton moved at constant speed with friction coefficient 0.3 requires a force of 29.4 N, doing 147 J of work over 5 m.
When force is variable, the concept extends through integration. Work done equals the area under the force–displacement graph. For instance, if F = 2x + 5, the work from x = 0 to x = 2 m is obtained by integration, giving 14 J.
Thus, Physics provides a precise, mathematical framework for understanding work, power, and energy, beyond everyday notions.