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Starting and stopping

Forces and motion · level 1, lesson 2 of 6

A question to start

A toy car sits on the floor. Will it start rolling on its own? And once it is rolling, why does it stop?

The idea

Things do not start moving on their own. A ball on the grass stays there until somebody kicks it. To start something moving, you need a push or a pull.

Stopping needs a force too. When you catch a ball, your hands push against it to stop it. When a bike stops, the brakes rub on the wheels. A toy car rolling on the carpet stops because the carpet rubs on its wheels. That rubbing is a force too, and it is called friction.

a push starts itrubbing slows it

If there were nothing to rub on and no air in the way, a moving thing would keep going for ever. Far out in space, that is just what happens: a spacecraft that is moving keeps moving, without its engines on, for years.

Worked examples

1. A ball on ice. A puck slides a long way on ice, because ice is slippery and rubs on it only a little. On grass, a ball stops sooner: grass rubs a lot.

2. Catching. To stop a ball, you push back on it with your hands. A hard throw needs a bigger push to stop.

3. A Mars rover. A rover on Mars stops when its motors stop turning its wheels and hold them still. It does not roll on by itself on flat ground.

Try it

  1. What do you need to start a ball rolling?
  2. Why does a rolling toy car stop on a carpet?
  3. Does a moving spacecraft far out in space need its engines to keep going?
  4. What do your hands do to stop a ball you catch?
Answers
  1. A push (or a pull).
  2. The carpet rubs on the wheels. The rubbing, called friction, is a force that slows it.
  3. No. With nothing to rub on, it keeps going.
  4. They push against it, the opposite way to how it is moving.

Common mistake

"Things stop because they run out of push." A moving thing does not carry a push inside it that gets used up. It stops because something else, like rubbing or a hand, pushes against it.

Where it leads