The potential energy diagram below is one possibility for getting a trapped car. On this diagram the maximum total energy level which the car can have and still remain trapped by two pairs of magnets is shown in Figure 5.
Notice that for some points of the diagram the total energy of the car equals its potential energy. According to the law of conservation of energy, the car cannot travel where the potential energy is greater than the total energy, because kinetic energy cannot have a negative value. Since the kinetic energy is always positive, the cart will stay in the region where potential energy is equal to or less then the total energy. At the locations where the potential energy and kinetic energy are equal, the car comes to a momentary stop, then starts moving in the opposite direction. The points where the cart momentarily stops are called turning points.
Look at Figure 5, where on the graph would the turning points be for the car that begins at 5 cm?
Identifying the turning points is a useful way of describing the region in which an object is restricted. Using potential and kinetic energy is an easy way to determine the turning points.
Once trapped, will the car stay there forever?
When an object is trapped, it must receive some energy if it is to get out of the trap. The energy diagram in Figure 6 represents a car that is trapped between two sets of magnets.
Supposed you wanted to help get this car out of its trap. How much energy would you need to give the car?
The energy needed to remove an object from a region in which it is trapped is called the binding energy. The binding energy is the difference between the object's total energy and the energy needed to set it free from its trap.
We discussed binding energy in Module A, you may wish to review this section now.