Rolling friction
Why are wheels different? When you push on most objects they either slide or fall over. When you push on a wheel, it does neither - it simply rolls along. Let us look more closely at what happens when we push objects - including wheels.

Image you have a rectangular block like a brick. You stand it on end, then push on the top. The brick tilts over until it loses its balance and falls over. When you take a smaller block that is shaped like a cube and push from one side it slides along the surface. If you try pushing a wheel you find that something different happens.

Imagine you have a wheel on a table. You push your finger across the top of the wheel. This makes the top of the wheel tip forwards but the wheel does not fall over. It rolls. This movement is due to the wheel's curved surface. If you look at the wheel from the side you can see that only a small area is in contact with the ground. You may expect the wheel to slide at this point when you push it but it doesn't. The surface just rolls over the ground.

The rolling action saves a great deal of energy. This can be seen by comparing how a heavy box can be moved without wheels and then with wheels. First think about pushing a heavy box across the floor without the use of wheels. As you begin to push, the box does not move. The force of friction rises to match your pushing force. Eventually you push hard enough to overcome the force of friction which is holding the box steady and the box begins to move. All the time you push on the box, a smaller force of friction, called sliding friction, pushes against you. It makes it hard for you to push the box. The moment you stop pushing the box, the sliding friction is the only force left acting on the box. The friction stops the box moving.

Now think about the box on a trolley. You push on the trolley gently. The trolley does not move because the friction between the ground and the wheels matches the force you are using. As you increase your pushing force the wheels begin to roll. When the wheels roll, each part comes into contact with the ground for a brief moment and then moves on. There is no sliding friction because the wheel does not drag its surface across the ground. This means that there is less friction to push against when wheels are used and so less energy is needed to do the pushing.

The first people did not use wheels. When they wanted to move a load they dragged it on a sled. A great deal of effort had to be made by the people or animals moving the sled because the sliding friction between the sled and the ground pushed against them. About six thousand years ago, people discovered that a heavy load could be moved by placing logs under it and rolling the load on top of it. Two thousand years later they discovered that wheels provided a more convenient way of moving loads and the first carts were invented.

Today wheels are used in many different ways. They are used by trucks, buses, cars and bikes. Even aircraft have them to help them take off and land. Your chair may have wheels. If it has, just push on the ground with your feet and feel yourself roll away.

Is there friction between a wheel and the ground?
Yes, there is but it is not as strong as sliding friction.

The surface of the wheel and the surface of the ground have microscopic ridges and grooves on them. They interlock where the surfaces meet. If the wheel is rolling along there is a small amount of friction as the wheel surface touches the ground.

Why does a wheel keep rolling when you are no longer pushing it?
Every moving object keeps on moving unless a force acts on it to slow it down and stop it. When you stop pushing a wheel, it keeps going because this is what moving objects do.

Does the friction between the wheel and the ground slow down the wheel and make it stop?
Yes, it does. It takes a wheel longer to stop than a sliding object because there is less friction acting on the wheel than the sliding object.

Can a wheel ever slide?
Yes, it can. When a wheel slides we say that it skids. If the driver of a car puts on the brakes too strongly the wheels stop turning and the surfaces in contact with the road slide along the road leaving black skid marks. The skid mark is made from pieces of the tyre that have been pulled away by the road surface.

How do you make the wheels turn on a bicycle?
When you press down the pedals on a bicycle the force passes along the chain to the gear on the back axle. This force makes the back wheel turn. As the back wheel rolls forwards the front wheel also rolls forwards as if pushed by the back wheel.

How does the back wheel push the bicycle forwards?
When you press down the pedals you make the back wheel push backwards on the road. The force of friction between the wheel and the road is strong enough to match this force and it pushes back in the opposite direction. This allows the wheel to move forwards. You can feel how the pushing force of friction helps your foot to move you forwards, if you walk very slowly. As you do this you will feel the pushing force pass along your foot from heel to toe.

Does the front wheel of the bicycle push on the ground in the same way?
No. It just rolls along. It is driven by the pushing force of the back wheel. The purpose of the front wheel is to support the front of the bicycle and allow it to move.

What makes a car's wheels turn?
The power of the engine. The engine produces a force, which acts on one of the pair of wheels. These wheels are called the drive wheels. If the car has drive wheels at the back it is said to have back-wheel drive. If the drive wheels are at the front the car is said to have front-wheel drive. This force from the engine pushes the wheel round and the force of friction acts in the opposite direction to help the wheel push the car forwards. The pair of wheels that are not attached to the engine simply support the car and are rolled along by the pushing action of the drive wheels. Some cars have four-wheel drive. All the wheels receive a pushing force from the engine. Four-wheel drive cars are particularly useful for crossing wet and muddy ground because the engine can push at four places on the ground to keep the car going rather than just two.