Raven draws two balls on a page: a small one and a big one, both with the same speed arrow.
"Same speed," she says. "Which carries more energy?"
"The big one," Rocket says. "More stuff moving."
Raven draws two more: the same ball twice, one with a short arrow and one with an arrow twice as long.
"Double the speed," she says. "Double the energy?"
Nova dims to a soft glow. "Careful," she says. "Yesterday's fast roll pushed the cup much more than twice as far."
Rocket stares at the arrows. "More than double? Speed counts extra?"
"What do you notice about how the cup slid?" Raven asks. Nova hums. "The standards have a sentence for this."
The science standards put it in one sentence. Kinetic energy is proportional to the mass of the moving object and grows with the square of its speed.
Proportional to mass means that doubling the mass at the same speed doubles the kinetic energy.
Grows with the square of its speed means something bigger. Doubling the speed makes four times the kinetic energy, because two times two is four.
A physics text gives an example. A car travelling at 100 kilometres per hour has four times the kinetic energy it has at 50 kilometres per hour.
This is why speed matters more than mass. A small fast ball can carry more energy than a big slow one.
| Change | Kinetic energy becomes |
|---|---|
| Double the mass, same speed | 2 times as much |
| Triple the mass, same speed | 3 times as much |
| Double the speed, same mass | 4 times as much |
| Triple the speed, same mass | 9 times as much |
Think about the cup. A ball at double speed does not just hit twice as hard. It also travels twice as far in each second.
A physics text adds one more point. The energy of a moving object does not depend on which way it moves, only on how fast.
Kinetic energy can never be less than zero. A stopped object has zero, and every moving object has some.
The standards give everyday examples. Ride a bicycle at different speeds, roll different sizes of rocks downhill, or compare a light plastic ball with a tennis ball.
A gentle toss and a hard throw of the same ball differ only in speed. The hard throw carries far more energy, which is why we use soft balls.
| Statement | True or false? |
|---|---|
| Kinetic energy is proportional to mass. | ? |
| Doubling speed doubles kinetic energy. | ? |
| Kinetic energy grows with the square of speed. | ? |
| A stopped object can have kinetic energy. | ? |
Strong reasoning. Tomorrow is Field Lab: a ramp, a cup and a tape measure will show you kinetic energy in centimetres.