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Science · Force and motion

Friction

Why do we slip on ice but not on a carpet? Find out that friction acts opposite to motion, depends on how rough the surfaces are, and in everyday life both helps us and causes problems.

  • 15 min
  • 2 simulations
  • Mini quiz

Updated:

Preview

The question

You have probably felt your foot slip while walking on an icy pavement in winter. In the same shoes, you walk easily on dry tarmac. What has changed? And if there were no friction at all, would life be easier, or would we be unable to walk, write, or even tie a knot?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Ordering

Order the surfaces by friction

Put the cards in order by dragging them or using the up and down buttons. When you have finished, submit your answer.

Prediction

Predict first

On what kind of surface do you think the toy car will stop in the shortest distance?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Friction and force

Get the block moving on the carpet

First choose the floor, then increase the pushing force in 1 N steps. Watch the friction force and resultant force readouts. When you find the smallest force at which the block starts to slide, submit your answer.

Prediction

Predict first

You push the same block on ice and on carpet. On which one do you need a bigger force to get it moving?

It doesn’t matter whether you’re right; the simulation opens once you choose.

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Frequently asked

4 questions

Why does friction produce heat?

When surfaces rub against each other, some of the energy of motion turns into heat. That is why your hands warm up when you rub them together quickly and why a match catches fire when struck. Machines are oiled to reduce this heating and wear.

Why is a heavy box harder to push on the same surface?

A heavy box presses harder on the surface; the tiny bumps and dips of the two surfaces catch on each other more tightly, and the friction force increases. That is why a full box is harder to push than an empty one on the same surface.

When measuring friction with a spring scale, why do we read it after the box starts sliding, not at the moment it starts?

The force needed to get a resting object moving is usually larger than the force needed to keep it moving. Once the box is sliding at a constant speed, the pull equals the friction, so the reading at that point shows the friction during motion.

Why do we slip on ice, and how do sand and salt help?

Ice is very smooth and often has a thin layer of water on top, which greatly reduces the friction between your shoe and the ground. Your foot cannot push back on the ground well, so you slip. Sand reduces slipping by making the surface rough, and salt by melting the ice.