School › Friction › Ages 10–11
Friction: the force against motion
Where friction comes from, why it is harder to start something sliding than to keep it going, and when friction helps or hinders. With a force meter fair test.
Read
Friction is a force between two surfaces that touch. It always acts in the opposite direction to the motion, or to the way something is trying to move.[3][1]
Close up, all surfaces are rough, with tiny bumps. Part of friction comes from the bumps catching, and part from the surfaces' molecules sticking to each other a little.[1][2]
It takes a bigger force to start something sliding than to keep it sliding. Friction on something that is not yet moving is called static friction; on something that is sliding it is kinetic friction.[2][1]
How much friction there is depends on the two materials. Rubber on dry concrete grips well, rubber on wet concrete grips less, and ice on ice is very slippery. A lubricant such as oil can cut the friction between steel parts to less than a tenth.[1][3]
Friction is often useful: it lets us walk without slipping. But it also wastes energy, turning movement into heat. Wheels help, because rolling friction is much smaller than sliding friction.[2][5]
Air also slows things down. This air resistance, or drag, is a kind of friction with the air. It depends on how fast the object moves through the air and on its shape.[7]
Try it: Fair test: which surface has the most friction?
You need
- A force meter (newton meter, 0–10 N)
- A shoe or a small box with a hook or string
- Some weights or books to put in the box (always the same)
- Different surfaces: table top, carpet, sandpaper sheet, kitchen foil, a towel
Safety
Pull gently; don't let the force meter snap back. Keep sandpaper on the table and wash hands after touching it. Don't use heavy loads.
What to do
- Attach the force meter to the shoe or box.
- Pull it slowly and steadily across the table top. Write down the force just as it starts to move.
- Keep pulling at a slow, steady speed and write down the force while it is sliding.
- Do the same on each of the other surfaces.
- Fair test: change only the surface. Keep the same shoe or box, the same load and the same slow speed.
- Repeat each surface three times and work out the average.
Play: More friction or less?
Does this change give more friction or less friction?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 15–16