You have been studying ideas of potential energy and kinetic energy (KE). Potential energy due to position in the earth’s gravitational field or the potential energy of a stretched or compressed spring are examples of mechanical potential energy (PE). Energy stored in a lump of coal that is released when it is burned is chemical potential energy and not mechanical potential energy. This is the subject of chemistry.

The sum of kinetic and mechanical potential energy is called total mechanical energy. If there is no friction. The total mechanical energy of a system is conserved or constant if forces such as gravity or spring forces are the only forces acting on the object. These are called conservative forces. When a rock is thrown up, down, horizontally, or at a weird angle and air resistance is ignored, the total mechanical energy of the rock stays constant, or KE + PE = constant. This also means KE can become PE and vice versa. If there is friction, the work done by friction “gobbles up” total mechanical energy. For example, if you jump off of a high diving board into a swimming pool, as you fall toward the water, your initial potential energy becomes kinetic energy, but your total mechanical energy is conserved until you hit the water.

Discussion Question: What happens to your total mechanical energy when you hit the water? Where does the lost mechanical energy go? Why do you fill a swimming pool with water before diving in? Can you think of another important example of non-conservation of total mechanical energy?

Now I want to make a very important point. Total mechanical energy is conserved if there is no friction or other non-conservative forces doing work. If non-conservative forces are doing work, then total mechanical energy is not conserved. Total energy is all energy, not just mechanical energy. Now here is the big lesson, total energy is always conserved!!! This is a fundamental law of nature.