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Wheels and rollers

Forces and motion · level 2, lesson 3 of 6

A question to start

Try to push a heavy suitcase across the floor by sliding it. Then tip it on to its wheels and push. Why is it so much easier?

The idea

When something slides, its whole bottom rubs along the ground, and friction pushes hard against it.

When something rolls on wheels, the wheel does not rub along the ground. Each bit of the wheel touches the ground, stays still for a moment, and lifts off again. Rolling is many times easier than sliding.

sliding: a lot of frictionrolling: very little

People knew this long ago. Builders moved huge stones on logs used as rollers. The wheel is one of the oldest inventions there is. Inside a bike or a skateboard wheel are tiny steel balls, called ball bearings, which let the wheel turn on its axle with very little rubbing.

Wheels still need some friction with the ground. On ice, a car's wheels can spin without moving the car. The wheel pushes back on the ground; friction lets the ground push the wheel forward.

On Mars, Curiosity and Perseverance each roll on six wheels, about half a metre across, each with its own motor.

Worked examples

1. A shopping trolley. Full of shopping, it is hard to drag on its side, but easy to push on its wheels.

2. Moving a fridge. Furniture movers slide a small platform with wheels, called a dolly, under a fridge so it rolls.

Try it

  1. Why is rolling easier than sliding?
  2. What did builders long ago use to move huge stones?
  3. What are ball bearings for?
  4. How many wheels does Perseverance have?
Answers
  1. A rolling wheel does not rub along the ground, so there is much less friction.
  2. Logs, used as rollers.
  3. To let a wheel turn on its axle with very little rubbing.
  4. Six.

Common mistake

"Wheels work because there is no friction." Wheels need the ground to grip them. Without friction a wheel just spins on the spot, like a car on ice.

Where it leads