KS3 · Physics

Friction

Slide in your socks on carpet, then on a polished floor: same you, same run-up, totally different slide. Something is fighting you, far harder on carpet. That's friction.

Physics · Forces

Where does friction act, and which way?

View: 1 · Small push. Showing 2 layers: Block and floor, Small push

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Explore

switch situations, then tap the parts ↑

You push the block gently to the right, but it doesn't budge. Friction is already there: it grips back with 4 N to the left, exactly matching your 4 N push. Friction doesn't wait for movement — it stops movement starting.

Four situations, one block. Watch where the friction arrow starts, which way it points, and how long it is. Force sizes are example values.

Zoom in

What two surfaces look like close up

View: Dry surfaces. Showing 2 layers: Dry surfaces, Zoom note

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compare the two views, then tap the parts ↑

Zoomed right in, even a floor that feels smooth is a landscape of tiny bumps and dips. The bumps on the block sit down in the dips of the floor, so the two surfaces are locked together at lots of points.

A magnified slice through the block and the floor where they touch.

The friction chain

Reason it through

Why does a sliding block slow down and stop?

Link 1 of 4

First link · your turn

Start with the surfaces. What are the block and the floor like, close up?

Locked — reveal the link above first

Physics · Friction

More friction or less?

Does each change increase or reduce the friction between the two surfaces?

Still to sort

Increases friction (0)

The surfaces grip each other more.

Where the line is: Rougher surfaces, or removing a lubricant.

Reduces friction (0)

The surfaces grip each other less.

Where the line is: Smoother surfaces, or adding a lubricant between them.

6 of 6 still to sort.

Every change below alters one pair of surfaces. Decide what it does to the friction between them.

Check your thinking

What's really going on?

You lean on a heavy box sitting on a shiny, smooth-looking floor and push gently. The box doesn't move at all.

Which idea is closest to what you think is happening?

There's no friction yet — friction only starts once the box is sliding.Friction is helping me, pushing the box the same way I am.The floor is smooth, so there's no friction. Something else must be stopping the box.Friction between the box and the floor is gripping back against my push, so the box can't start to slide.

How sure are you?

I'm sureI think soI'm guessing

There's no friction yet — friction only starts once the box is sliding.

This is a common misconception

Tempting, but friction is already working. If there were no friction at all, even your gentle push would set the box sliding. The box stays still because friction grips back against your push. Friction doesn't only slow movement down — it can stop movement starting.

Friction is helping me, pushing the box the same way I am.

This is a common misconception

If friction pushed the same way as you, the two forces would add together and the box would shoot off. It doesn't move, so something must be acting the other way. That's friction: it acts against your push, trying to stop the box sliding over the floor.

The floor is smooth, so there's no friction. Something else must be stopping the box.

This is a common misconception

Smooth to your eye isn't smooth close up. The floor and the bottom of the box are both covered in tiny bumps, and those bumps grip. A polished floor usually gives less friction than a rough one, but not none.

Friction between the box and the floor is gripping back against my push, so the box can't start to slide.

This matches the evidence

Exactly right. Friction acts where the box meets the floor, against the direction you push. Your gentle push isn't big enough to beat it, so the box stays put. Push harder and it may start to slide — but friction keeps acting against the movement.

Friction acts between two touching surfaces, against the sliding (or the attempt to slide) — whether or not anything is moving yet.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why things slow down, why some things won't start moving, and how a drop of oil changes everything.

What you need to know

A force is a push or a pull that comes from two objects acting on each other. Friction is one kind of force.
Friction is the force of one surface gripping or rubbing on another.
Friction acts where two surfaces touch, against them sliding over each other.
Friction prevents movement or slows it down.
Friction is caused by the roughness of surfaces: tiny bumps that catch on each other.
A lubricant is a substance added between two surfaces to reduce friction.
Forces, including friction, are measured in newtons (N).

The big picture

Friction is a force: one surface gripping or rubbing on another. It acts where two surfaces touch and works against them sliding over each other, so it slows movement down or stops it starting at all. It happens because surfaces are rough — covered in tiny bumps, even when they look smooth — and those bumps catch on each other. Like every force, friction is measured in newtons (N). A lubricant such as oil, added between the surfaces, keeps the bumps apart and reduces friction.

Key points

1Friction = one surface gripping or rubbing on another.

2It acts at the contact surface, against the sliding — or against a push that hasn't got anything moving yet.

3Cause: rough surfaces. Tiny bumps catch and grip, even on surfaces that look smooth.

4Effect: movement is slowed down or prevented.

5Rougher surfaces usually mean more friction; smoother surfaces or a lubricant (like oil) mean less.

6Forces are measured in newtons (N).

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Worked example

Problem

Two identical hockey pucks are flicked with exactly the same push. One slides across a rough carpet; the other slides across a smooth, polished wooden floor. Which puck travels further, and why?

⚠ Watch out

Drawing the friction arrow on a sliding object pointing the same way it moves. On a block sliding across a floor, friction points backwards, against the movement — that's why the block slows down.

🧠

Memory hook

Think of friction as Velcro you can't see: two surfaces covered in tiny hooks and bumps that catch on each other. Roughen them and they grab harder. Slip oil between them and the hooks can't reach each other.

✓

Check yourself

Without looking back: explain in three linked steps why a book pushed across a desk slows down and stops. Then name one change that would let it slide further, and say why it works.

Flashcards

(12)

What is friction?

The force of one surface gripping or rubbing on another.

What causes friction?

The roughness of surfaces: tiny bumps on each surface catch and grip on each other.

What does friction do to movement?

It prevents movement or slows it down.

Where does friction act?

Where the two surfaces touch, along the contact surface.

A block slides to the right across a floor. Which way does friction on the block point?

To the left — against the block's movement.

Can friction act on something that isn't moving?

Yes. Push a heavy box gently and friction grips back against your push, so the box doesn't start to move.

What is a lubricant?

A substance added between two surfaces to reduce friction — for example oil or grease.

How does a lubricant reduce friction?

It sits between the surfaces and keeps their bumps apart, so they can't grip each other as much.

What unit are forces measured in?

Newtons, written N.

What is a force?

A push or a pull that comes from two objects acting on each other.

Does a surface that looks perfectly smooth have friction?

Yes. Close up it is still bumpy. Smoother surfaces usually give less friction, but not zero.

Give one way to increase friction.

Make a surface rougher, e.g. spreading sand on an icy path or using grippier soles.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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More friction or less?

What's really going on?

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