Rocket jogs a lap of the lawn, then sprints one. He flops down, breathing hard.
"The sprint felt like way more than twice the work," he says.
Raven checks the Field Log. "Twice the speed, four times the kinetic energy. Your legs had to supply all of it."
Nova circles overhead. "Where does a ball's kinetic energy come from in a game?" she asks.
"The player," Rocket says. "A kick, a throw, a swing. The player gives energy to the ball."
"And where does it go when the ball stops?" Raven asks. "What do you notice when you catch a hard throw?"
Rocket rubs his palm. "My hand. It stings and gets warm."
Nova dips. "Then you already know next week's story."
Every moving ball in a game got its kinetic energy from somewhere. A kick, a throw or a swing transfers energy from the player to the ball.
A faster kick gives the ball more kinetic energy, and the energy grows with the square of the speed. A little more speed means a lot more energy.
When a ball is caught, the energy moves again. Some goes into the hands and some becomes warmth and sound.
A physics text adds that friction removes some of the energy a person puts in and converts it to thermal energy. Nothing is lost, only changed.
Mass and speed both count. That is why the standards ask you to compare them separately: mass at a fixed speed, then speed at a fixed mass.
| Sport moment | Where the kinetic energy comes from | Where it goes |
|---|---|---|
| A kicked ball flies | The player's leg | Drag in the air, then the ground or a catch |
| A rolled ball hits a cup | The hand that rolled it | The cup, then friction with the floor |
| A runner sprints | The runner's muscles | Friction, drag and warmth |
| A caught ball stops | The throw | The hands, warmth and sound |
| Week review | True or false? |
|---|---|
| Kinetic energy is the energy of motion. | ? |
| Doubling mass at the same speed doubles kinetic energy. | ? |
| Doubling speed doubles kinetic energy. | ? |
| Energy is measured in joules. | ? |
| A slow heavy package always has more kinetic energy than a fast light ball. | ? |
Great week. Tomorrow is Game Day: a family cup slide contest. Next week, energy that waits: potential energy.