Q.What is a Joule?

Work, Energy and Power: Definitions, Forms and Units

These three quantities describe how much a force accomplishes and how fast. They are distinct but tightly linked, and keeping their definitions and units straight is the foundation of the whole chapter.

Work

Work is done when a force moves its point of application. For a constant force $F$ producing a displacement $s$ at an angle $\theta$ between them,

Work is a scalar; its SI unit is the joule ($1\text{ J} = 1\text{ N·m}$). Note $W$ can be zero (when $\theta = 90^\circ$) or negative (when $\theta > 90^\circ$, e.g. friction).

Energy

Energy is the capacity to do work, measured in the same unit, the joule. Two mechanical forms recur constantly:

• Kinetic energy (energy of motion): $\;K = \tfrac{1}{2}mv^2$
• Potential energy (energy of position, near Earth's surface): $\;U = mgh$

The work–energy theorem ties work to motion: the net work done on a body equals its change in kinetic energy, $W_{net} = \Delta K$.

Power

Power is the rate of doing work (or transferring energy):

$P = \frac{W}{t} = \vec{F}\cdot\vec{v}$ …