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Friction is a force between a pair of touching surfaces, never something one object has on its own.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will describe friction as a force between two named surfaces, say which way it points, measure it at a steady speed, and explain why the same object needs a different push on different surfaces.
You know that every force is a push or a pull between two things, and that two forces along a line either balance or leave a resultant. Friction is one of those forces, and it obeys both rules — including the one about two things, which is the part almost everyone forgets for this particular force.
You also know that a steady speed means balanced forces. That gives you a way to measure friction: slide something at a steady speed, and the pull you need is equal to the friction.
| Term | What it means |
|---|---|
| Friction | The force between two touching surfaces when one slides, or tries to slide, over the other. |
| Surface | The outside of something, where it meets the world. |
| Pair of surfaces | Two surfaces touching. Friction is always about a pair, never one. |
| Rough | A surface with lots of bumps that catch on another surface. |
| Smooth | A surface with few bumps, so it slides more easily over another. |
| Grip | Friction that stops a surface from sliding at all. |
| Lubricant | Something slippery, like oil, put between surfaces to reduce friction. |
A class wants to know how much friction a wooden brick has. They pull it with a spring scale until it slides at a steady speed, and write down the reading. Then they do it again on a different floor. Then again.
| What the brick slid on | Spring scale reading |
|---|---|
| a rubber doormat | 12 N |
| a wooden floor | 7 N |
| a sheet of ice | 3 N |
Before reading on, answer the question they set out to answer: how much friction does the brick have?
You cannot, and the readings are why. The brick was the same brick all three times — same wood, same mass, same shape, nothing added and nothing taken away — so no single number in that column can be a fact about it. Three different answers came out of one unchanged object.
So try the other suspect: perhaps the number belongs to the floor. Here is one more reading, taken on the doormat that gave 12 N:
| What slid on the doormat | Spring scale reading |
|---|---|
| the wooden brick | 12 N |
| a steel can | 5 N |
The doormat did not change either. So the number is not a fact about the floor.
It is not a property of the brick and it is not a property of the floor. Then what is it a property of? Say what would settle it: what would you have to change, and what would you have to hold still, for the reading to stay the same? The next part gives the answer that survives all four readings.
Friction is a force. So, like every other force, it has two ends: the two surfaces that are touching.
That is why you cannot answer how much friction does a brick have — the question is missing half of itself. You can answer how much friction is there between this brick and this doormat, and you get a different answer for the same brick on ice.
Friction always pushes back against the sliding. If the sled is going right, the friction on it points left. It does not have a favorite direction of its own; it simply opposes whatever is happening.
And friction can be measured. Slide the object at a steady speed with a spring scale. A steady speed means the forces are balanced, so the pull on the scale is exactly equal to the friction between the two surfaces.
Another way: action
Rub your palm along a table, then along your sleeve, then along a window. Same hand every time. Three different amounts of grip — so the grip was never a fact about your hand.
Another way: steps
Even surfaces that look smooth are covered in tiny bumps and dips, far too small to see. When two surfaces touch, their bumps catch on each other, a little like two hairbrushes pressed together bristle to bristle. To slide one over the other, the bumps have to be dragged past each other, and that takes a push.
This is why friction depends on both surfaces. A rough doormat has big bumps, but so does the bottom of a brick. Put the brick on ice, and the ice's surface is much flatter, so there is less for the brick's bumps to catch on. Put a smooth steel can on the doormat, and now the doormat's bumps have less to grab. Change either surface and the catching changes, which is why the number always belongs to the pair.
Stack a second brick on top of the first and slide them together on the same floor. The spring scale reading doubles. Stack a third and it triples.
| Bricks stacked | Push needed on ice |
|---|---|
| 1 | 3 N |
| 2 | 6 N |
| 3 | 9 N |
The extra bricks press the bottom brick harder onto the ice, so the bumps of the two surfaces are squeezed together more tightly and catch more. The pair of surfaces is still the same pair — brick on ice — but pressing them together harder makes more friction between them. That is why a full shopping cart is harder to push than an empty one, and why a loaded truck needs longer to stop.
Friction is not a nuisance to get rid of. Without it you could not walk: your shoe pushes back on the ground, and friction between the sole and the ground is what stops the shoe slipping. Car tires grip the road the same way, and the brakes on a bicycle work by rubbing pads against the wheel.
Sometimes friction gets in the way. It makes machines warm up and wear out, and it makes heavy boxes hard to slide. Then people choose a different pair of surfaces: they put wheels under the box, oil between the metal parts of an engine, or wax on the bottom of skis. Every one of these changes the pair of surfaces that touch, which is the only way to change friction.
Friction is one force among several, so it follows the balanced-forces rule. When a box slides at a steady speed, the push forward and the friction backward are balanced. When the push is bigger than the friction, the box speeds up. When nobody pushes, friction is the only force along the floor, so the box slows down and stops.
That last case explains something people have wondered about for thousands of years: why do moving things stop? Not because they run out of force — they never had any force in them. They stop because friction between them and the ground pushes back. On ice, where the friction is small, a hockey puck glides a very long way before it stops.
Basketball shoes have soft rubber soles with a zigzag pattern. On a clean wooden gym floor, the friction between the soles and the floor is large, so a player can stop and change direction without sliding. A shoe company tests this by pulling a shoe across a floor sample with a force gauge.
Suppose a shoe with a weight pressing down on it needs about 60 N to slide across a clean gym floor. If the floor is dusty, the dust gets between the surfaces and the pull falls to about 35 N, a drop of $60 - 35 = 25$ N. That is why players wipe the bottoms of their shoes during a game. The shoe did not change and the wood did not change. The dust changed what was touching what.
The same shoe on an icy sidewalk might need only 10 N. Nobody would say the shoe lost its friction on the way from the gym to the sidewalk. The pair of surfaces changed, and so did the force between them.
A family is moving a dresser across a carpeted bedroom. Pushing it at a steady speed takes about 300 N, so the friction between the dresser's wooden feet and the carpet is about 300 N. That is a hard push for one person.
Movers slide plastic furniture sliders under each foot. Now the pair of surfaces is plastic on carpet, and the push drops to about 100 N, a third as much. If they empty the drawers first, the dresser presses on the carpet less hard, and the push falls further, perhaps to 60 N.
Both tricks work by changing the friction between the pair. The sliders change what touches the carpet. Emptying the drawers changes how hard the two surfaces are pressed together. Nobody changed the dresser's friction, because the dresser never had any of its own.
The sentence almost every child writes is sandpaper has a lot of friction, or ice has no friction. Teachers say it too, as shorthand, and it is the single most stubborn wrong idea in this part of physics.
Here is what goes wrong with it. If friction lived in the sandpaper, then two sheets of sandpaper face to face would have twice as much, and a sheet of sandpaper in an empty room would still have its friction sitting there, doing nothing, waiting. Neither makes sense, because friction is not a thing to have. It is something that happens between two surfaces when they touch.
The fix is a habit of speech. Never say the friction of X. Say the friction between X and Y. If you cannot name the Y, you have not been told enough to answer.
A second mistake is thinking friction only happens while something moves. A heavy box that you push gently and that does not budge has friction on it too: the floor pushes back exactly as hard as you push, which is why nothing moves. That grip is friction between the box and the floor, keeping them from sliding at all. Push harder and the grip grows to match, until at last the box breaks free and slides.
Read the first measurement.
$\text{brick on doormat: } 12\ \text{N}$
Pulled at a steady speed, so the pull equals the friction.
Read the second measurement.
$\text{brick on ice: } 3\ \text{N}$
The same brick, a different floor.
List what stayed the same.
$\text{the brick}$
Same wood, same mass, same shape.
Find how much they differ.
$12 - 3 = 9\ \text{N}$
A big difference from one unchanged brick.
Say whose numbers they are.
$\text{brick-on-doormat and brick-on-ice}$
Two pairs of surfaces, one number each.
Name the two surfaces.
$\text{the sled's runners and the snow}$
Friction always has two ends.
Find the direction of sliding.
$\text{to the right}$
The rope pulls it that way.
Point friction the other way.
$\text{friction on the sled: to the left}$
Friction pushes back against the sliding.
Check the case of pushing back.
$\text{sliding left} \Rightarrow \text{friction right}$
Friction has no favorite direction; it opposes the motion.
Check the case of no sliding.
$\text{nobody pulls} \Rightarrow \text{no friction}$
With nothing trying to slide, there is nothing to oppose.
State the rule.
$\text{friction opposes sliding}$
Opposing, not choosing.
Read the first floor's measurement.
$\text{push } 20\ \text{N on carpet, steady speed}$
A steady speed means balanced forces.
Find the friction on the carpet.
$\text{friction} = 20\ \text{N}$
Balanced forces are equal and opposite.
Name the new pair.
$\text{box on a polished wooden floor}$
A new pair gives a new friction: here, 8 N.
Compare the push with the new friction.
$20\ \text{N forward against } 8\ \text{N back}$
The push stays the same.
Take the smaller from the bigger.
$20 - 8 = 12\ \text{N}$
Opposite forces subtract.
Say what happens.
$\text{the box speeds up}$
The forces are unbalanced on the smoother floor.
Find the push for a steady speed.
$\text{push} = 8\ \text{N}$
To stop speeding up, the pusher must ease off until the forces balance again.
Name what kind of thing friction is.
$\text{a force}$
And every force needs two things.
Find the missing thing.
Rewrite the question properly.
It takes $18$ N to keep a rubber-soled shoe sliding along a dry sidewalk, and only $6$ N to keep the *same* object sliding along a wet tiled floor. What changed between the two tries?
Complete the worked solution: one of a wooden brick needs $3$ N to keep sliding on a sheet of ice. A student stacks two identical ones, then three, and slides each stack on the same surface. Find the push each stack needs.
Double the push for two.
$2 \times 3 =$ a N
Two press down twice as hard on the same surface.
Triple the push for three.
$3 \times 3 =$ b N
Three press down three times as hard.
Compare three with the rough surface.
$\text{three stacked} - 12\ \text{N}$ on the rough surface $=$ c N
A heavier load on a smooth floor can rub as hard as a light one on a rough floor.
Each of these forces acts between two things. Match the description to the force.
| friction | weight | drag | tension | |
|---|---|---|---|---|
| acts between two touching surfaces and pushes back against sliding | ||||
| acts between the Earth and everything near it, pulling downward | ||||
| acts between a moving thing and the air or water it is moving through | ||||
| acts between the two ends of a stretched rope |
Four sentences about a wooden block sliding on sandpaper. Mark the one that says it properly.
This task has no paper form; do it on a device.
A student slid a sack of sand at a steady speed, first on a gravel path with a push of $24$ N and then on a sheet of plastic with a push of $10$ N. Fill in the friction, in newtons, between each pair of surfaces.
| friction between the pair (N) | |
|---|---|
| a sack of sand on a gravel path | |
| a sack of sand on a sheet of plastic |
a sack of sand needs $24$ N to keep sliding on a gravel path and $10$ N on a sheet of plastic. How many more newtons does the first pairing need?
Answer: unit: N / kN
This sled is being pulled to the right across snow. Click the arrow that stands for the rubbing between the sled's runners and the snow.
This task has no paper form; do it on a device.
A student pushes a wooden brick along a rubber doormat with a steady push of $12$ N, and it slides at a steady speed. It then slides onto a sheet of ice, where the friction between the two is only $3$ N. The student keeps pushing with $12$ N. What is the resultant force on it now, in newtons?
Answer: unit: N / kN
A climber's rubber shoe grips dry rock so well she can stand on a ledge the width of a coin. The same shoe, on wet ice, slides from under her. Why?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A student slid a sack of sand at a steady speed, first on a gravel path with a push of $24$ N and then on a sheet of plastic with a push of $10$ N. Fill in the friction, in newtons, between each pair of surfaces.
| friction between the pair (N) | |
|---|---|
| a sack of sand on a gravel path | |
| a sack of sand on a sheet of plastic |
You can name both surfaces in any friction question and say which way the friction points. Tell someone why ice has no friction is the wrong way to say it.
16. Your turn: why is *how much friction does this shoe have?* a question with no answer?, step 2
$\text{what the shoe is on}$
The question names only the shoe.
16. Your turn: why is *how much friction does this shoe have?* a question with no answer?, step 3
$\text{friction between the shoe and the rock}$
On rock or on ice are different questions with different answers.