echoesof.science · Ages 10–11
Friction: the force against motion
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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.
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.
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.
Experiment: Fair test: which surface has the most friction?
- 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.
| Surface | Force to start (N) | Force while sliding (N) | Average sliding force (N) |
|---|---|---|---|
| Table top | I think … will need the biggest force, because … | ||
| Carpet | I think … will need the biggest force, because … | ||
| Sandpaper | I think … will need the biggest force, because … | ||
| Foil | I think … will need the biggest force, because … | ||
| Towel | I think … will need the biggest force, because … |
Questions
Draw a box being pulled to the right. Draw arrows for the pull and for friction.
🍎Why is it harder to start pushing a heavy box than to keep it moving?
In your fair test, what did you change, what did you measure and what did you keep the same?
Give one example where friction is useful and one where we try to reduce it.
Sources for every fact on this sheet: echoesof.science/school/friction/sources/