Potential Energy

Gravity provides a nice example of work done by a force that can be considered Potential Energy. Potential Energy is sometimes written as PE or V. Most common is the notation of the text where Potential Energy is written as U. How much work does the force of gravity do on an object of mass m as the mass moves from initial height yi to final height yf?

Wg = (Fg) s = (m g) s = ( - m g j) ( yf - yi ) j = - m g yf + m g yi

We could even get carried away with this and write a more general form of the displacement s,

Let s = ( xf - xi ) i + ( yf - yi ) j

Then the work done by gravity is

Wg = (m g) s = ( - m g j) [( xf - xi ) i + ( yf - yi ) j ] = - m g yf + m g yi

The horizontal component of the displacement does not make and difference!

In either case, we have

Wg = - m g yf + m g yi

This means we can define the gravitational potential energy as

Ug = m g y

and then we can write the work done by the force of gravity as

Wg = - ( Uf - Ui )

Notice the order of these subscripts,

Wg = - U = - (Uf - Ui)

U -- the change in potential energy -- is the final value minus the initial value.

Wg = - U


Conservative Forces
Return to ToC, Ch8, Conservation of Energy

(c) Doug Davis, 2001; all rights reserved