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Physics Force and motion
Class 6 Beginner About 10 min Premium

At rest or moving? It depends who is watching

Watch a passenger in a school bus, first from the roadside and then from a seat in the bus. Is the passenger moving? Is the tree? Find out why the answer depends on where you watch from.

Core concept

In short: Decide whether things are still or moving, depending on what you compare them with.

An object is at rest if it does not change its position compared with what is around it, and moving if it does; so rest and motion depend on what we compare with

Topic: Force and motion

We say something is moving when its position changes compared with the things around it, and at rest when its position stays the same. But compared with what? Think of a passenger sitting in a school bus. Someone standing by the road sees the passenger whizz past: the passenger is moving. The friend in the next seat sees the passenger stay right beside them: the passenger is at rest! Both are right, because they are comparing with different things. That is why we say rest and motion are relative. Look out of a bus window and the trees even seem to run backwards.

Key words explained

What you will learn

  • Explain what it means for an object to be at rest or in motion.
  • Explain why the same object can be at rest for one watcher and moving for another.
  • Describe how the trees by the road seem to move when seen from a moving bus.
  • Work out the speed of a walking passenger as seen from the bus and from the road.

Before you start

  • Know that speed tells us how fast something moves, for example in km/h.

Materials

  • A bus ride, or a toy car with a small figure on it

Equipment

  • A road with a tree beside it

Procedure

  1. Watch the passenger from the roadside as the bus passes at 40 km/h, and record.
  2. Watch the same passenger from a seat in the bus, and record. Look at the tree too.
  3. Stop the bus and watch from the roadside, and record.
  4. Let the passenger walk forward in the moving bus; watch from the bus and from the road, and record both.

How this simulation is simplified

  • The bus moves at a steady speed along a straight road.
  • A walking passenger walks forward at 4 km/h compared with the bus.
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