There are many energy conversions between potential and kinetic energy as the cars travel around this double looping roller coaster.

### Conservation of Energy

The **law of conservation of energy** states that within a closed system, energy can change form, but the total amount of energy is constant. Another way of expressing the law of conservation of energy is to say that energy can neither be created nor destroyed. An important part of using the conservation of energy is selecting the system. In the conservation of energy just as in the conservation of momentum, the system must be closed. In a **closed system**, objects may not enter or leave, and it is isolated from external forces so that no work can be done on the system.

In the analysis of the behavior of an object, you must make sure you have included everything in the system that is involved in the motion. For example, if you are considering a ball that is acted on by gravity, you must include the earth in your system. The kinetic energy of the ball considered by itself may increase and only when the earth is included in the system can you see that the increasing kinetic energy is balanced by an equivalent loss of potential energy. The sum of the kinetic energy and the potential energy of an object **is the total energy of the system.** As potential energy goes down kinetic energy goes up and visa versa

#### Summary

- In a closed system, energy may change forms but the total amount of energy is constant.
- In a closed sytem nothing can enter or leave.
- Total energy = potential energy + kinetic energy

#### Practice

The following video demonstrates Newton Ball tricks. Use this resource to answer the questions that follow.

http://www.youtube.com/watch?v=JadO3RuOJGU

- What happens when one ball is pulled up to one side and released?
- What happens when three balls are pulled up to one side and released?
- What happens when two balls are pulled out from each side and released?