Editing Transfer Of Energy (section)

== What to Say ==

Begin by explaining that when the tennis ball is being held above the ground, it has potential energy (PE). Explain that PE is stored energy, related to height above the ground, and that it is changed into kinetic energy (KE) as the ball falls toward the ground. Say that KE is energy of motion (i.e. a moving object has KE). This is why the ball moves and gets faster as it falls. The farther it falls, the more PE is converted to KE and, therefore, the faster it moves. You can drop the tennis ball to show them this. Explain that you are providing PE to the ball when you lift it.

Next, explain conservation of energy. Say that energy can’t be lost, only transferred between two objects (i.e. two balls colliding which I’ll show you shortly) or changed into different forms of energy (i.e. PE to KE which we just saw). Explain that since energy is conserved, the tennis ball can’t bounce higher than the initial height that it was dropped from. If it went higher, its final total energy would be greater than its initial PE which was its initial total energy. The conservation of energy says that the initial and final total energy need to be equal.

Now, you can show them the demo using both balls. It should be easy to see that the tennis ball goes higher than its initial height. Ask if anyone knows why that happened. Wouldn’t it be breaking the law of conservation of energy? If no one answers correctly, remind them about what you said earlier about energy being transferred between objects (otherwise, something such as “That’s right; remember earlier I said…”). Now, explain that when the soccer ball hits the tennis ball, it transfers its energy to the tennis ball. This gives the tennis ball more energy than it initially had. However, the soccer ball now has less energy than it originally had. Explain that conservation of energy still holds. The total energy of both balls combined has stayed the same from beginning to end. You just changed the amount of energy in each ball.