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Smooth and rough

Forces and motion · level 2, lesson 2 of 6

A question to start

Slide a toy car down a board covered in kitchen foil, then down a board covered in a towel. Which goes farther? What about in socks on a polished floor, compared with on a rug?

The idea

Different surfaces rub by different amounts.

  • Smooth, slippery surfaces, like ice, polished floors and wet tiles, have little friction. Things slide a long way on them.
  • Rough surfaces, like sandpaper, carpet, grass and rubber, have lots of friction. Things stop quickly on them, and feet grip.
smooth: slides farrough: stops soon

We often want friction, and choose rough surfaces on purpose. Shoes have rubber soles with patterns. Tyres have treads. Roads are rough, not polished. On icy days, people put grit or sand on the paths.

Other times we don't want it. Ice skates have smooth blades. Slides are polished. Oil in a bike chain makes the parts slide past each other easily.

On Mars, the wheels of the rover Curiosity have zigzag treads, called grousers, to grip the dusty ground.

Worked examples

1. The foil and the towel. The car rolls farther on the foil. The towel's fibres catch on the wheels and slow them.

2. A tug of war. The team in grippy trainers can pull harder without sliding than a team in smooth-soled shoes.

Try it

  1. Name two surfaces with little friction and two with lots.
  2. Why do trainers have patterns on their soles?
  3. Why do people put sand on icy paths?
  4. Why are there grooves on a rover's wheels?
Answers
  1. For example ice and polished wood (little); carpet and sandpaper (lots).
  2. To grip the ground: more friction.
  3. To make the path rougher, so feet grip and do not slip.
  4. To grip the dusty, sandy ground of Mars.

Common mistake

"Smooth things have no friction." Even very smooth surfaces rub a little. Ice has very little friction, but not none: a puck slows down in the end.

Where it leads