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Moving energy

Energy and power · level 2, lesson 1 of 6

A question to start

A ping-pong ball rolls into a tower of blocks, and nothing happens. A bowling ball rolls into it at the same speed, and the tower crashes down. Why can one moving ball do so much more than the other?

The idea

Everything that moves has energy, just because it is moving. This is called movement energy, or by its proper name, kinetic energy.

A moving thing has more kinetic energy if it is:

  • heavier: a bowling ball has far more than a ping-pong ball at the same speed;
  • faster: a ball thrown hard has more than the same ball rolled gently. Speed matters a lot: twice as fast means four times the energy.
light: little energyheavy: lots of energy

When a moving thing hits something, it can pass its energy on: knocking blocks over, making a dent, making a bang, or warming things up a little.

On Mars, a ball rolling at a certain speed has exactly the same kinetic energy as on Earth. Moving energy depends on how heavy something is and how fast it goes, not on gravity. So a cart rolling into you on Mars hits just as hard as it would on Earth.

Worked examples

1. A car and a bike. At the same speed, a car has far more kinetic energy than a bike, because it is much heavier. That is why it needs much bigger brakes.

2. Twice as fast. A ball rolling twice as fast has four times as much energy. It can knock over four times as many blocks, or roll four times as far up a slope.

Try it

  1. What do we call the energy of a moving thing?
  2. Which has more kinetic energy: a lorry or a car, at the same speed?
  3. A ball goes twice as fast. How many times more energy does it have?
  4. Does a rolling cart have less kinetic energy on Mars than on Earth, at the same speed?
Answers
  1. Kinetic energy (movement energy).
  2. The lorry: it is heavier.
  3. Four times.
  4. No, the same: kinetic energy does not depend on gravity.

Common mistake

"Twice as fast means twice the energy." Kinetic energy grows faster than speed: twice the speed gives four times the energy, and three times the speed gives nine times.

Where it leads