Summary

To maintain conservation of energy the potential energy must be negative in the region near the magnet. In fact, the shape of the potential and kinetic energy diagrams turned out to be identical, although inverted. If we add the dip of the potential energy and the hump of the kinetic energy together, they will cancel out and the resulting diagram will be a straight line. This line represents the total energy of the car which does not change if friction is not present.

The graph of potential energy vs. distance which you created are examples of potential energy diagrams. These diagrams can be useful in describing motion for all types of objects. As you can see from the answers to the questions above, once you have the potential energy diagram for a situation, you can describe the motion of an object.

The usual method of using potential energy diagrams is to:

• start with the physical situation
• use the physics to draw the potential energy diagram
• describe the motion

In this way you can describe the motion even when you have never seen it. We can also describe other features in addition to speed and acceleration. In later activities we will look at some of those features.

The potential energy diagram provides an alternative way to describe motion of objects such as cars. They can also be used for analyzing the motion of very small objects --- ones we cannot see. Thus, we need to understand first their value for objects we can see before using them with objects that are too small to see.