Friction can help or hinder movement — Engineering, 7–10 years
Friction is a contact force that engineers sometimes need and sometimes try to reduce. Engineering, 7–10 years.
What friction does
Friction is a rubbing force between two surfaces that touch. It makes a sliding object slow down, but it also lets your shoes grip the ground and lets a pencil mark paper. Rough surfaces usually give more grip than smooth ones.
Why engineers think about friction
People needed carts, wheels and tools to move without wasting too much effort, yet they also needed feet, brakes and knots to hold firm. Engineers learned to add oil or smooth surfaces when they wanted less rubbing, and rough tread when they wanted more grip. The problem decides whether friction is useful.
Testing two surfaces
Put the same toy car on a smooth table and then on a towel. Give it the same gentle push each time. It rolls 120 centimetres on the table but only 45 centimetres on the towel. The towel creates more friction, so the car loses its movement sooner.
The reasonable mistake
A common thought is that engineers should always remove friction because it slows things down. That sounds sensible when you imagine a wheel rubbing against its axle. But without friction, a bicycle could not push against the road, a brake could not stop it, and you could not stay standing.
Friction in daily life
Bike tyres have patterned rubber to grip the road, while oil inside moving parts helps them rub less. Climbing shoes and football boots use special soles for extra grip. Engineers choose the amount of friction for the job: enough to control movement, but not so much that parts wear out quickly.
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