Monday
Monday: The Missing Height
Rocket holds a tennis ball at shoulder height above a patio tile and lets it drop.
It bounces back up to about his knee. Raven holds the tape measure upright beside the bounce.
"It came back lower," Rocket says. "Every time. Where did the rest of the height go?"
Raven writes the two numbers. "What do you notice? It never comes back to your hand. Not even close."
Nova hovers level with the top of the bounce. "Energy does not disappear," she says. "So something else must have changed."
Rocket presses his palm flat on the tile where the ball landed. "Warmer? I cannot feel it."
"Too small to feel," Raven says. "But you heard the thump. Sound is energy too."
Nova hums. "You just found the week's rule."
Energy is never lost
A physics text states the law of conservation of energy. The total energy is constant in any process.
Energy may change in form or be transferred from one system to another, but the total remains the same.
That rule is why the missing height is a puzzle worth solving. The ball had stored energy at your hand. Less came back as height.
The rest did not vanish. It changed form. Some became sound, the thump you heard. Some became thermal energy, a tiny bit of warmth.
The science standards say it this way. When the motion energy of an object changes, there is always some other change in energy at the same time.
| Moment | Where the energy is |
|---|---|
| Ball held at shoulder height | Stored as gravitational potential energy |
| Ball just before it hits | Kinetic energy |
| Ball squashed on the tile | Elastic energy in the ball, plus some sound and warmth |
| Ball at the top of the bounce | Less potential energy than at the start |
| The missing part | Thermal energy in the ball and tile, and sound in the air |
- Read the question.
- Tap your answer.
Thermal energy: warmth is motion too
A physics text explains that atoms and molecules inside all objects are in random motion. This internal energy from random motion is called thermal energy.
Thermal energy is related to the temperature of the object. More random jiggling means warmer.
When the ball squashes against the tile, some of its motion becomes faster jiggling of particles in the ball and tile. They get a tiny bit warmer.
The same text says that when surfaces rub, their atoms vibrate and that energy is converted into heat. That is why rubbing your hands warms them.
The amounts in a bounce are too small to feel, but they are real. Scientists track them with an energy inventory, a list of before and after.
| Statement | True or false? |
|---|---|
| The total energy stays the same even when it changes form. | ? |
| A dropped tennis ball bounces back to exactly its drop height. | ? |
| Thermal energy is the random motion of particles inside an object. | ? |
| The missing bounce energy was destroyed. | ? |
Stop and let them cool. Where did the energy for the warmth come from?
Write "rubbing" and "thermal energy" in your Field Log with your answer.
Drops are from standing reach only. A patio, hallway or kitchen floor works, with a tape measure held against a wall.
The energy facts come from OpenStax physics texts hosted by LibreTexts and from the Next Generation Science Standards.
Strong start. Tomorrow you will follow a bounce and a sliding stop step by step and meet a real bounce number.