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Fast and slow

Rockets and landing · level 1, lesson 3 of 6

A question to start

A snail, a bicycle, a car, an aeroplane and a rocket. Put them in order from slowest to fastest. How fast does a rocket have to go to get to space?

The idea

Speed is how far something goes in a certain time. A fast thing goes a long way in a short time; a slow thing goes a little way.

Thing About how far it goes in one second
A snail 1 millimetre (the width of a grain of sand)
You, walking 1 metre (one big step)
A bicycle 5 metres
A car on the motorway 30 metres
A jet plane 250 metres
A rocket going round the Earth 8,000 metres (8 kilometres)
A ship on its way to Mars (going round the Sun) about 30,000 metres

A rocket in orbit goes about 8 km every second: from one side of a city to the other in a couple of seconds. It goes right round the whole Earth in an hour and a half.

To get that fast, a rocket has to keep pushing for a long time. A big push for a short time makes a small change; a big push for a long time makes a huge change. A rocket's engines push for about eight to ten minutes to reach orbit.

Worked examples

1. Racing toy cars. Push two toy cars, one gently and one hard. The hard push makes the faster car.

2. A heavy and a light car. Give the same push to a toy truck full of blocks and an empty one. The empty one goes faster. Heavy things need a bigger push to get going.

Try it

  1. Which is faster, a bicycle or a jet plane?
  2. About how far does a rocket in orbit go in one second?
  3. You push an empty cart and a full cart the same. Which goes faster?
  4. How can a rocket get so fast?
Answers
  1. The jet plane, by a lot.
  2. About 8 km.
  3. The empty cart.
  4. Its engines keep pushing hard for many minutes.

Common mistake

"Rockets go fast because they are big." Big rockets are big because they carry lots of fuel, so they can keep pushing for a long time. A tiny rocket with a lot of fuel could go fast too.

Where it leads