Friction is a force between two surfaces that are sliding, or trying to slide, past each other.
Friction A force between surfaces that opposes sliding.
Step 1: Something to Notice
Watch first. The explanation comes later.
A pull of 2 units shifts a block easily on ice. The identical pull on carpet does not move it a millimeter.
The pull was the same both times. What is the carpet doing that the ice is not?
Step 2: Find Out
Hold the pull at 3 and change only the surface.
Step 1 — Predict
The same pull of 3 on ice, wood and carpet. Where will the block go furthest?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: Find Out
On carpet, turn the pull up one unit at a time until the block finally shifts.
Step 1 — Predict
A pull of 2 on carpet does nothing. What does that tell you about the friction?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 4: So Here Is Why
Now the explanation, after you have seen it happen.
Always backwards
Friction always acts opposite to the movement. Push the block right and friction pushes left.
Rougher means more
Carpet gives much more friction than ice, so much less of your pull is left over.
You can measure it
Find the smallest pull that just moves the block. The friction is about that size.
Friction is a force you can measure
It acts between two surfaces sliding past each other, and it always opposes the motion. You cannot see it, and you can see what it does — the trolley stops sooner on carpet than on wood.
surface The outside of something, the part you can touch.
Friction is often wanted
Brakes, shoe soles, tires and gripping a pencil all depend on friction. It is not simply a nuisance to be reduced; much of the time it is the thing that makes something work.
Step 5: A Common Mistake
Lots of people think
“Rough surfaces just slow things down. That is a property of the surface, not a force.”
Step 6: Friction Is Not the Enemy
The same idea somewhere new.
Without friction you could not walk, hold a pencil, or stop a bicycle. Engineers spend as much effort adding friction — brake pads, tire tread, grip tape — as removing it.
Step 7: Think It Through
Practice makes it stick.
The Sticky Box
Problem 1 of 2
A pull of 4 does not move a box. A pull of 5 just barely does. About how big is the friction?
units
Left Over
Problem 2 of 2
You pull with 7 on a surface where the friction is 2. How much force is left to move the box?
units
Friction
1 of 4
Which way does friction act on a block sliding to the right?
2 of 4
Same pull on ice and on carpet. Which goes further?
3 of 4
You pull with 9 and the friction is 3. What is the leftover force?
units
4 of 4
Name something that would be impossible without friction.
Step 8: Quick Check
Show what you know.
Question 1 of 1
Is friction a force?
Step 9: Explain It in Writing
Claim, evidence, then reasoning.
The question
Using your runs on ice, wood and carpet, write an argument answering the question: what is the carpet doing to the block, and which way does it act?
Evidence you can use
- With the pull held at 3, the block went 150 cm on ice in 5 seconds.
- With the same pull of 3, it went 100 cm on wood.
- With the same pull of 3, it went only 25 cm on carpet.
- The pull stayed on 3 for all three surfaces. Only the surface was changed.
- On carpet, a pull of 1 and a pull of 2 did not move the block at all.
- On carpet, a pull of 3 was the smallest pull that moved it.
- Brakes, shoe soles and tires all work because of friction.
Words to usefrictionforcebackwardssurfaceleft overthe same
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- Friction is a force between surfaces that are sliding.
- It always acts against the movement.
- Rougher surfaces give more friction, so less of your pull is left over.
- You can measure it by finding the pull that just beats it.