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Rubbing slows things down

Forces and motion · level 2, lesson 1 of 6

A question to start

Rub your hands together fast. What do you feel? Now slide a book across a table. Why does it stop?

The idea

When two things rub together, each one pushes against the other's movement. This rubbing force is called friction. It always works against the way things are sliding.

The book slides one way across the table. Friction from the table pushes it the other way. So the book slows down and stops.

sliding this wayfriction pushes back

Friction also makes things warm. That is why your hands feel hot when you rub them. A spacecraft coming down through the air gets very hot for a similar reason: the air rubs and squashes against it.

If you look at even a smooth table under a strong magnifier, it has tiny bumps. As two things slide, their bumps catch on each other. That is a big part of friction.

Worked examples

1. A slide at the park. You go down a metal slide fast. In shorts on a hot rubber slide, you go slowly, because rubber on skin rubs a lot.

2. Stopping a bike. The brakes squeeze rubber pads against the wheel. Friction between the pads and the wheel slows the wheel down. Squeeze harder, and it slows faster.

3. On Mars. The wheels of a rover would spin in place on the ground if there were no friction. Friction between wheel and ground is what lets the wheels push the rover along.

Try it

  1. What is the name for the rubbing force between two things?
  2. Which way does friction push on something that is sliding?
  3. Why do your hands get warm when you rub them together?
  4. Would a rover be able to drive if there were no friction at all?
Answers
  1. Friction.
  2. Against the way it is sliding.
  3. Friction between them turns the movement into heat.
  4. No: its wheels would spin without gripping.

Common mistake

"Friction is always bad." Without friction you could not walk, hold a pencil, or stop a bike. Friction is a nuisance when we want things to slide, and a help when we want them to grip.

Where it leads