**Work against inertia**

When you apply a force on a stationary but freely moving object, you are working against its inertia or tendency to remain stationary. This also applies to changing the velocity or direction of an object. The work done on a freely moving object only occurs over the distance **while you are applying the force**.

For example, if you throw a ball, the work done consists of the distance you accelerated the ball until you let it go. Once you have thrown the ball, it will continue at a constant velocity (minus the effect of air resistance) and no further work is done.

Another example of work against inertia is the work done by the force of gravity, when you drop an object from some height. Since the force of gravity is **F = mg**, where **m** is the mass of the object and **g** is the acceleration of gravity, the work done in dropping an object from a height **h** is **W = Fd = mgh**.

**Note** that the equation **W = mgh** is the same as for the potential energy of an object at some given height: **PE = mgh**.

## Comments

No comments have yet been made