Grade by Grade
C. Matter and Energy · Forces Causing Movement

Friction: The Rubbing Force

C2.1 C2.3 A1.2About 35 minutes
Learning goals
  • I can explain that friction is a contact force made when two surfaces rub together.
  • I can explain that friction slows down or stops moving things.
  • I can predict which surfaces make more friction and test my prediction fairly.
  • I can give examples of when friction is helpful and when it is a problem.

Words to know

friction
A contact force made when two surfaces rub together. It pushes against the way something is moving.
surface
The outside or top layer of something, like the top of a table or the bottom of a boot.
rough
Bumpy, not smooth. Rough surfaces usually make more friction.
smooth
Even, with no bumps. Smooth surfaces usually make less friction.
grip
To hold on without slipping. Friction gives grip.
prediction
What you think will happen, and why, before you test it.

Slip and slide

In winter, your boots walk easily on a dry sidewalk. But step on a patch of smooth ice and... whoa! Your feet slip.

What is different? It is the amount of friction.

A snowy sidewalk where one part is smooth, shiny ice and the part beyond it is covered with brown sand.
Smooth ice has very little friction, so boots slip. Sand makes the surface rough and adds friction so people can walk safely. AI-generated image

What is friction?

Friction is a contact force. It happens when two surfaces rub against each other.

  • Friction pushes against the way something is moving. So it slows things down and can stop them.
  • Rough surfaces (carpet, sandpaper, gravel, the bumpy bottom of a boot) usually make more friction.
  • Smooth surfaces (ice, a waxed floor, a slide) usually make less friction. Things slide easily on them.
  • Friction also makes heat. Rub your hands together fast. Feel them get warm!
movingfrictionramp
A toy car rolls off a ramp. It moves to the right. Friction pushes the other way, to the left, and slows it down until it stops.
Friction helps us...Friction can be a problem when...
Boots with bumpy soles grip the snow.It makes heavy boxes hard to slide across the floor.
Sand on an icy sidewalk stops people slipping.Rubbing wears out shoes, tires and the knees of pants.
Bike brakes rub on the wheel to stop the bike.A rusty bike chain is hard to pedal.
Hockey gloves and stick tape help hold the stick.A dry door hinge squeaks and sticks.
Winter tires have special treads to grip snow and ice.It slows down a sled you want to go fast.
Friction can be helpful or a problem

Changing friction

  • To get more friction: add a rough surface (sand on ice, grippy treads, rubber mats).
  • To get less friction: make it smooth or slippery (wax on skis, oil on a bike chain), or use wheels so things roll instead of rub.
Worked example

Let's read a friction test

Jayden rolled the same toy car down the same ramp onto three floors. He let go each time without pushing. He measured how far the car rolled after the ramp.

  • wood floor: 150 cm
  • carpet: 40 cm
  • bath towel: 25 cm

Which surface had the most friction? Which had the least?

  1. First check it was a fair test: same car, same ramp, same height, no push. Only the floor surface changed. Yes, it was fair.
  2. More friction slows the car down sooner, so it rolls a shorter distance.
  3. The shortest distance is 25 cm, on the towel. So the towel had the most friction.
  4. The longest distance is 150 cm, on the wood floor. So the smooth wood had the least friction.

The towel had the most friction. The wood floor had the least. Rough, fuzzy surfaces made more friction than the smooth surface.

How far the toy car rolled after the rampwood floor150 cmcarpet40 cmbath towel25 cm050100150distance (cm)
Jayden's results as a bar graph. The longest bar (wood) means the least friction. The shortest bar (towel) means the most friction.

Science talk

Share your results like a scientist: "I predicted the car would go farthest on the wood floor because it is smooth. My results showed it went 150 cm on wood and only 25 cm on the towel, so my prediction was correct."

Just for fun

Why do skaters never tell secrets on the ice?

Because there is so little friction that things just slip out!

Hands-on activity

Ramp and roll friction test

You need: a toy car, a long hardcover book or a piece of cardboard for a ramp, 2 or 3 books to prop it up, a tape measure or ruler, and three surfaces (for example a hard floor, a rug and a towel spread flat).

  1. Make a ramp by leaning the long book on the stack of books.
  2. Predict: on which surface will the car roll the farthest? Why?
  3. Put the end of the ramp on the first surface. Hold the car at the top and let go (no pushing!).
  4. Measure from the bottom of the ramp to where the car stops. Do it 3 times and write down each distance.
  5. Do the same on the other surfaces. Keep the ramp, the height and the car the same.
  6. Make a chart. Which surface had the most friction? Was your prediction right?

Bonus: rub your hands together for 10 seconds. How do they feel? Now put a drop of hand lotion on them and rub again. Does it feel different?

What to look for: the car goes farthest on the smoothest floor and stops soonest on the fuzziest surface. Hands get warm when rubbed; lotion makes them slide more easily (less friction).

Your turn

1

What is friction?

2

Does a rough surface or a smooth surface usually make more friction?

3

Why do people sprinkle sand on icy sidewalks?

4

Name one way friction is helpful.

5

Name one way friction can be a problem.

6

Rub your hands together quickly. What happens? Why?

7

A puck slides farther on smooth ice than on a rough sidewalk. Why?

More practice: warm-up

1

Is friction a contact force or a non-contact force?

2

Which is more slippery: ice or carpet?

3

What does friction do to a moving object?

4

Do wheels make it easier or harder to move a heavy box?

5

What do skiers put on their skis so they slide faster?

More practice: on your own

1

Why do winter boots have deep, bumpy treads on the bottom?

2

A sled goes very fast on an icy hill but slowly on a hill with soft, fresh snow. Explain using friction.

3

In Jayden's test, the car went 150 cm on wood and 40 cm on carpet. How much farther did it go on wood?

4

Lia tested two floors, but she pushed the car hard on one floor and let it go gently on the other. Can she tell which floor has more friction? Why?

5

A bike chain squeaks and is hard to pedal. What could you do? Why does it work?

Challenge

1

You are pushing a heavy box of books across a carpet and it is hard. Give two ways to make it easier, and explain each using friction.

2

Hockey players want their skates to slide easily, but they want their gloves to hold the stick tightly. Explain how friction is helpful in one place and a problem in the other.

3

Predict: will a toy car roll farther on a bath mat or on a baking sheet? Explain your prediction, then tell how you could test it fairly.

Check your understanding

Quiz

1.

Friction happens when...

2.

What does friction do to a rolling toy car?

3.

Which surface has the least friction?

4.

Why put sand on an icy path?

5.

Which is a helpful use of friction?

6.

A car rolled 30 cm on a rug and 120 cm on a tile floor. Which surface had more friction?

For parents and teachers

Big idea: Friction is a contact force between two surfaces that rub. It acts against motion, slowing or stopping objects (linking C2.1 and C2.3). Rough surfaces usually give more friction; smooth surfaces less. Friction is sometimes helpful (grip, brakes) and sometimes a problem (wear, extra effort). The ramp test practises the experimentation process (A1.2): predict, keep it fair, measure several times and compare.

How to help: Notice friction in winter: why sand or grit goes on sidewalks, why boots have treads, why sleds go faster on icy hills. Ask your child to predict, then check.

Common difficulties:

  • Thinking friction only happens on rough things. Even smooth surfaces have some friction; it is just less.
  • Mixing up distance and friction. A shorter roll means more friction. Have your child say it as "stopped sooner = more friction".
  • Pushing the car. For a fair test the car should be released, not pushed, so each trial starts the same way.

Safety: test only on clear, flat floors. Oil and lubricants should be handled by an adult.