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Pushes make things go

Rockets and landing · level 1, lesson 1 of 6

A question to start

A toy car sits on the floor. It does not move by itself. What do you have to do to get it rolling? And once it is rolling, what makes it stop?

The idea

Things do not start moving on their own. Something has to push or pull them. Scientists call a push or a pull a force.

  • A push or a pull can start something moving: you push the car and off it goes.
  • It can stop something: you put your hand in front of the rolling car.
  • It can turn something: you push the car from the side and it changes direction.
  • A bigger push makes a bigger change. Push the car gently and it rolls slowly; push it hard and it zooms.
pushit rolls

Why does a rolling car stop, even when nobody touches it? Because there are hidden pushes: the floor rubs on its wheels, and the air pushes against it. These are called friction and air resistance. On a smooth floor the car rolls a long way; on a thick carpet it stops quickly.

A rocket is a machine that pushes itself. Instead of a hand, it has an engine that pushes it along.

Worked examples

1. Kicking a ball. A kick is a push. A soft kick sends the ball a little way; a big kick sends it far.

2. Catching a ball. Catching is a push too: your hands push against the ball to stop it.

3. A swing. Each push from behind makes the swing go higher. A pull on the ropes can stop it.

Try it

  1. What is a force?
  2. Name three ways a push can change how something moves.
  3. Why does a rolling ball stop on grass faster than on a smooth floor?
  4. What does the pushing for a rocket?
Answers
  1. A push or a pull.
  2. It can start it, stop it or turn it (and speed it up or slow it down).
  3. The grass rubs on it more: there is more friction.
  4. Its engine.

Common mistake

"Things slow down by themselves." They slow down because something pushes against them: friction or the air. Far out in space, where there is nothing to rub against, a moving thing just keeps going.

Where it leads