Which material grips best?
Some people enjoy rock climbing. They put on boots with a very good grip, clothes, which are hard-wearing and a helmet to protect their head. Most rock climbers climb in groups. When the group reach a rock face or cliff that they wish to climb, they rope themselves together. The leader then climbs a little way up the rock face then knocks in a short metal rod with a metal ring attached to it. The rope connecting the leader to the other climbers is put through the ring and the leader climbs a little further up. Another metal bar and ring is knocked into the rock and the leader climbs on. The other members of the team then begin to climb. They follow the leader up the cliff face to the top. The climb is only possible because of friction.

When a rock climber makes that first step up onto the cliff face the surface of the sole of the boot comes into contact with the surface of the rock. If you were to look at both surfaces through a microscope, you would see that they are covered in ridges and grooves. When a rock climber presses their boot onto a rock, the two surfaces become very close together. The ridges of one surface sink into the grooves of the other.
We move our legs with our muscles and bones. When a rock climber makes a step up onto a rock, some muscles in their leg tighten and pull on the bones. This makes the foot push downwards. The sole of the climbing boot receives this pushing force and a frictional force develops between the surface of the sole and the rock. If the frictional force matches the force pushing downwards, the boot stays in place and the climber can swing up the other leg to find a footing further up the cliff. If the frictional force cannot match the pushing force, the ridges come out of the grooves and a sliding frictional force develops. This is not as strong as the force from the leg pushing down and the boot slips.

Imagine you are a rock climber. You have climbed carefully up the rock face and almost reached the top. You are the last one in the group to finish the climb. You are standing on a rock ledge and can see the countryside rolling away behind you. Large birds such as crows and jackdaws fly below you. It is time to take those last few moves to the top. You put your foot on a piece of smooth rock but it feels firm. You lift your other leg so that all your weight is pushing down on your boot and the smooth surface. Your weight is too strong a force for friction to match and your boot begins to slide quickly. You lose your balance and fall backwards. You wave your arms about and somehow your hands touch the rope. You wrap your fingers round the rope. The ridges and grooves in the surface of your skin and the surface of the rope interlock. The force of friction between them is strong enough to stop your hands slipping and you grip the rope firmly while the rest of the group pull you to the top.

Friction helps rock climbers enjoy their sport. It can also help them survive if they lose their grip for a moment.

Do rock climbers use their hands?
Yes, they do. When a rock climber grips a rock the grooves and ridges in the skin of their fingers interlock with the grooves and ridges in the surface of the rock. This allows a strong frictional force to develop when climbers need to pull themselves up.

Do rock climbers use their hands and feet together?
Yes, they do. In the story you had put all your body weight on one foot and this was too much for the frictional force between your boot and the smooth rocky surface. What you should have done to reach the top on your own was also to be holding onto the rock face with your hands. If you had been doing this you would have had the frictional forces between your hands and the rock helping to support you. This would have given you a chance to move your boot to another position where the force of friction could have matched the force pushing down on your boot.

How are the metal bars in the rock held in place?
By friction. The ridges and grooves in the surface of the metal interlock with the ridges and grooves in the rock. When a metal bar receives a pulling force a frictional force develops between the metal and rock surface to match it. This means that the metal bar holds firm in the rock.

What are the metal bars for?
If a person fell off the rock face, the rope in the ring would stop them falling to the ground and the weight of the climber would be matched with the force of friction between the bar and the rock. The person could then swing on the rope to reach the rock face and start climbing again.

Are nails held in the wall by friction?
Yes, they are. The ridges and grooves in the surface of the nail interlock with those in the surface of the hole. Friction holds screws in walls too. When people talk about what would happen if there were no friction they do not think about nails and screws. If there were no friction nails would pop out of cupboards and the cupboards would fall to bits. Screws holding shelves on a wall would pop out into the air and the shelves would collapse.

Why do shoes need a good grip?
When you take a step you keep one foot on the ground and swing the other foot forward. The foot on the ground needs to stay in place so you can swing the other foot and make your stride. When you swing one leg forwards the foot of the other leg pushes backwards on the ground. Your shoe needs a good grip so that a strong force of friction can develop between the surface of the shoe and the ground to match the pushing force of your foot. If the frictional force is not strong enough your foot will slide backwards. This will make you take a smaller step or, if the slide is rapid, it may make you lose your balance.

When I stand still is friction holding me in place?
You are held in place by two other forces. Your weight presses down through your feet onto the ground and the ground presses back at your feet to hold you in place. Friction is created when a force develops to push the surface of your foot across the ground. When you take a step a force develops to push backward on the foot on the ground and a frictional force develops to try and match it.