Rocket rolls a tennis ball along the patio. It slows, wobbles and stops near Raven's feet.
"Nothing hit it," he says. "It just gave up."
Raven picks it up. "Balls do not give up. What do you notice? It was rolling, and now it is not."
Nova hovers over the spot where the ball stopped. "Its motion energy went to zero," she says. "So what else changed?"
"The patio got warmer," Rocket says. "And the ball. Friction."
Raven nods and taps the Field Log. "Yesterday's law. The energy changed form instead of vanishing."
"So a rolling stop and a bounce are the same story," Rocket says. "Just different amounts."
Nova hums. "A physics text even gives a number for the tennis ball bounce."
A physics text calls friction a force that changes mechanical energy into thermal energy. Mechanical energy means kinetic energy plus potential energy.
When a ball rolls to a stop, friction between the ball and the ground does the work. The ball's kinetic energy becomes thermal energy in both surfaces.
The text describes a player sliding on grass. Friction stops the player by converting kinetic energy into other forms, including thermal energy.
The same thing happened to your paper cup last week. It slid, friction acted, and it stopped slightly warmer than it started.
Drag from the air does the same job on a flying ball. The air and the ball warm up by a tiny amount.
The physics text describes a ball dropped on a hard floor. It compares the bounce height with the drop height.
For a new tennis ball on a tennis court, the text gives a bounce number of 0.85. The number compares the speed after the bounce with the speed before.
A perfectly bouncy ball would score 1 and come back to its drop height. Real balls score less than 1.
The text says the bounce height ratio is this number squared. Squaring 0.85 gives about 0.72, so the ball returns to about 72 percent of its drop height.
The other 28 percent of the energy became thermal energy in the ball and the court, and sound. The inventory still adds up.
| Energy before the drop | Energy after the bounce |
|---|---|
| Stored energy at the drop height: 100 percent | Stored energy at the bounce top: about 72 percent |
| Thermal energy in the ball and floor, plus sound: about 28 percent | |
| Total: 100 percent | Total: 100 percent |
| Statement | True or false? |
|---|---|
| Friction changes mechanical energy into thermal energy. | ? |
| A real tennis ball bounces back higher than it was dropped. | ? |
| The missing bounce energy becomes thermal energy and sound. | ? |
| The energy inventory adds up to less after the bounce than before. | ? |
Strong reasoning. Tomorrow is Field Lab: you will measure bounce heights on three floors and build your own energy inventory.