What is friction?
In 1977, a spacecraft was launched from Earth. It was called Voyager 2 and its mission was to visit the four largest planets in the Solar System. By 1979 it had reached Jupiter, the nearest large planet to Earth. Two years later it passed by Saturn, the planet famous for its rings. In 1989 Voyager 2 passed by the blue planet called Uranus and another large blue planet called Neptune. Since then Voyager 2 has moved out from the Solar System and is now crossing deep space.

You may think that it must have had a very powerful engine but in fact it was only equipped with small rocket motors and they no longer work. Despite this lack of power the spacecraft hurtles through space and may reach another star in about 80,000 years. Like all objects it was given a push to get it moving. This was provided by its rocket motors. Once it had received its pushing force it just kept on going. The reason it has never stopped is that it moves in a place where there is almost no friction.

To see how important friction is on the Earth, imagine that Voyager passes a piece of space rock coming the other way. The rock has been in space since early planets in the Solar System collided and broke up. Now it is heading towards the Earth. It has a cold surface and moves at great speed. As the rock reaches the outer atmosphere of the Earth, changes begin to take place. It begins gradually to slow down and its surface becomes slightly warmer. As it sinks into the atmosphere it slows down more and more and its surface becomes hotter and hotter. At first the surface glows red then becomes white hot. As the rock shoots across the sky it leaves a white trail of light we call a shooting star.

Eventually the rock is travelling so slowly that it no longer glows and it falls to the ground. The rock bounces when it hits the ground, then rolls. As it rolls it slows down even more. Eventually it stops rolling and begins to slide. It slows down more as it begins to slide and soon it is still. The force which has brought this billions-of-years-old space traveller to a halt is a force called friction.

The space rock first came under the force of friction at the outer atmosphere. This atmosphere is made from a mixture of gases. Each gas is made from tiny particles, which can only be seen by the most powerful microscopes. When the rock passed into the atmosphere these particles rubbed against its surface and began to slow it down. When surfaces rub together heat is produced. Just rub your hands together now and feel how they warm up straight away. As the rock fell through the atmosphere it rubbed on so many particles they made the rock hot enough to give out light. The particles also made the rock slow down.

When the rock rolled along the ground it met more frictional forces. As the rock moved along one part of its surface after another touched the surface of the ground. At the place where the two surfaces met a frictional force acted on the rock and slowed it down. When the rock stopped rolling and its surface slid on the ground another frictional force pushed against the rock to make it finally stop.

In our world, frictional forces are everywhere. They make things stop moving but they also provide grip. Imagine that someone out walking discovered the piece of space rock and pushed it with a stick. If they pushed it gently it would not move because the force of friction would be holding it in place. If they pushed more strongly the force of friction would not be strong enough to hold the rock and it would start moving again. As soon as the person with the stick stopped pushing, the rock would stop again. While the rock could travel billions of kilometres in space without a push it could only travel a few centimetres on Earth - due to the force of friction.

What did Voyager 2 do when it visited the planets?
It took pictures of them and recorded details of their atmospheres. The pictures and details were sent back as radio messages to Earth.

How can Voyager 2 move without rocket motors?
Its rocket motors once gave it enough of a push to start it moving and it has kept moving ever since. Once something is moving it will continue to move until another force acts on it. Friction is the force which acts on moving objects to slow them down and stop them. In deep space there are very few particles of gas to rub on the side of the spacecraft and no hard surfaces so the Voyager just keeps going.

Do all objects in space behave like Voyager 2?
Yes. Any object, which has received a push, will just keep going until it is acted on by another force. The planets, including the Earth, keep moving round the Sun because there is no friction to slow them down.

How were the planets first pushed?
The planets are part of the Solar System. It is thought that the Solar System formed in the following way. A star exploded near a huge cloud of gas and dust. The cloud began swirling round and the Sun formed at the centre. The dust stuck together to form lumps of rock. The lumps of rock stuck together to form planets which were given their push by the swirling cloud. They have kept moving ever since - for over four and a half billion years.

How does friction stop a rock moving when you push it gently?
If you could look at the surface of the ground and the surface of the rock with a powerful microscope you would see grooves and ridges on both surfaces. The ridges on one surface fit into the grooves on the other surface. When you push the rock gently, the ridges simply push against the walls of the grooves they are in and the rock stays still. If you push harder the ridges are raised out of the grooves and slide along the top of them.

How does friction stop a sliding object?
The grooves and ridges rub against each other. This makes a frictional force, which pushes in the opposite way to the way the rock is sliding. This pushing force slows the rock down and eventually the ridges settle in the grooves, lock together and the rock stops moving.