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?
Sample Solution