The Force That Slows Things Down: Friction!

Imagine you are running on grass, then suddenly you step onto a patch of ice. What happens? You slow down on the grass but slide wildly on the ice! Have you ever wondered why? Today we are going to find out about the invisible force that grips your feet to the ground — and why it sometimes disappears.

Friction is a force. A force is a push or a pull that can change how something moves. Friction acts between two surfaces that are touching and moving against each other. It works in the opposite direction to movement — so it slows things down or stops them from sliding. Every surface, even one that looks smooth, is covered in tiny bumps and dips that you cannot see with your eyes alone. When two surfaces rub together, those tiny bumps catch on each other. That catching is friction. The rougher a surface is, the more bumps there are to catch, so there is MORE friction. A smooth surface has fewer bumps catching, so there is LESS friction. This is why it is easy to slide on ice (very smooth) but hard to slide on carpet (very rough). Friction can be useful or unhelpful. It is useful when it stops you slipping on the pavement, helps your bike brakes work, or lets you write with a pencil on paper. It is unhelpful when it slows down a sledge on snow or makes machine parts wear out faster. Engineers and scientists think carefully about when to increase friction and when to reduce it.