Getting moving
Stand up. Lift one leg so your foot rises off the ground. Now move your raised foot forwards. Notice that your body tips forward and the pressure of the foot still on the ground changes. It moves forwards from heel to toe. When you put your foot down, raise the other one and walk as slowly as you can. You should feel the pressure of the feet move from heel to toe. When the pressure reaches the front of your foot it pushes backwards on the ground. It is met by the force of friction.

When two surfaces like the sole of your shoe and the ground meet, the microscopic ridges and grooves on the surfaces interlock in a similar way to the pieces in a jigsaw puzzle. At the place where there is a force pushing one surface along another, a friction force develops. This is caused by the way the ridges and grooves hold together. The friction force is as strong as the pushing force but acts in the opposite direction. When you take a step it is the frictional force that holds your foot in place on the ground.

What happens when someone stands on a banana skin? The underside of a banana skin is coated with the flesh of the banana. This banana flesh contains water. When a foot is placed on a banana skin the skin is pressed into the ground. When this happens, the flesh is squashed against the surface of the ground. The water in the flesh fills the microscopic grooves in the surface. This makes it impossible for ridges in the surfaces pressing down to enter the grooves. When you move your foot across a banana skin and the force at the front of your foot pushes backwards, there is no strong force of friction to push in the opposite direction and the foot slides. If you are walking slowly and tread on a banana skin you may just wobble and keep your balance but if you are running you may lose your balance completely and crash to the ground.

Sliding friction is no use to people who are walking and it can be a danger. That is why in supermarkets and other places a sign is put out when the floor has been recently cleaned. It warns people that the floor is wet and that there is a danger of slipping.
Sliding friction, however, has its uses. Think of the people who explore the snowy wastes at the North and the South Poles. They use sliding friction to move themselves, their food and their equipment. Everything is strapped to a sled. There are two metal strips underneath a sled which are called runners. They are designed to make sliding friction useful.

Imagine you are a polar explorer. You have a sled and a team of husky dogs to pull it. When you are ready to set off, you load the sled. All the time you are loading, the sled stays in one place. The weight of the sled pushing down on the ground is balanced by the ground pushing upwards. When everything is strapped onto the sled you gather your dogs and link them together with rope. You attach the last pair of dogs to your sled. When you are ready, you climb on the back of the sled and shout to the lead dog to move. It pulls on its rope and a small force of friction develops between the runners and the ground. This force is strong enough to hold the pull of one dog. When the other dogs see the lead dog pulling they start to pull too. Gradually their pulling force increases and is matched by the frictional force so the sled does not move. Eventually all the dogs are pulling so strongly that the frictional force cannot match them and the sled begins to slide. The moment the sled moves the force of friction falls and the dogs need to pull much less strongly to keep the sled moving. You are off on your way across the snow.

What makes the forces in the body?
The forces are made by muscles. They create their force because they have an unusual property. They can make themselves shorter or they can contract. When a muscle contracts it makes a pulling force. Muscles are attached to bones and bones are attached to each other at joints. Muscles are arranged on bones so that they are attached to two bones across a joint. Just feel behind your knee as you are sitting there. You should feel some cords running from your thigh to the back of your lower leg. These cords are called tendons and they attach some of your thigh muscles to your lower leg. When you make these thigh muscles contract, they pull on the lower leg and make it swing backwards under your seat. When you stand up and walk, other muscles in your leg contract and make your foot push backward on the ground.

Can people be hurt when they slip on a banana skin?
Yes, they can. They may suffer from sprains or they may fall and bruise themselves. If the fall was very heavy a bone may be broken. Banana skins should not be put on the floor for a joke because they can cause injuries.

Can other fruits cause people to slip?
Any fruit which is juicy can cause people to slip. In fact any substance which contains a liquid can cause people to slip. The liquid prevents the ridges and grooves from interlocking properly and allows them to be pulled over each other quite easily.

Are there two types of friction?
Yes, there are. There is the friction which holds something in place. Something which stays in one place is said to be static, so this holding friction is sometimes called static friction. The second kind of friction occurs when one surface is sliding over another. This is called sliding friction.

Is sliding friction like having no friction?
No, it is not. In sliding friction the ridges and grooves on one surface rub and bounce over the ridges and grooves on another surface. When they touch they still make a force which acts in the opposite way to which the object is pulled. If there were no friction the ridges and grooves would not touch at all.

Is there a way of separating the two surfaces so that there is no sliding friction between them?
Yes, there is. In a hovercraft a cushion of air is made between the hovercraft and the ground and the hovercraft moves on a cushion of air. Air is made from particles of gas which rub on the surface and create a small amount of friction so even in a hovercraft the movement is not totally without friction.