The Bowling Ball Pendulum
The Demo: A volunteer stands with their nose to a bowling ball that is suspended from the ceiling, and then lets go. Will she be safe when it comes back?
Quick Physics: Conservation of Energy tells us that the bowling ball will not go higher than its starting height (without some external force pushing on it).
A swinging bowling ball illustrates a very important principle of physics: The Conservation of Energy. The total amount of energy an object has stays the same, unless you do something to change it. To change the energy of something, you have to move it (for example, give it a push). The bowling ball starts with a certain amount of potential (stored) energy. It gets this energy because someone had to lift it up to hang it. Bowling balls are heavy and you have to do work to get it up in the air. Whenever we do work on something, we are giving it energy. When you let go of the ball, it swings downward like a pendulum. As it starts swinging, the energy changes from potential energy to kinetic, or moving, energy. The total amount of energy, moving plus stored, stays the same; it only changes form. When the ball swings back to where it started, the energy changes back to potential energy. Since the total energy has to stay constant, the kinetic energy of the ball must be zero and the ball must stop moving. It can’t hit you!
An important qualifier: if you push the ball instead of just letting go, you give the ball some extra kinetic energy. This extra energy makes it swing back farther than when it started. When it comes back, you better duck!