Spreading a Force Out
- Physics
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- Pressure is force divided by the area it acts over.
- The same force spread over more area does less damage.
- Concentrating a force into a small area is how cutting and piercing work.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Same weight, different footprint
Walk across deep snow in ordinary boots and you sink to your knees. Strap on snowshoes and you walk across the top. You have not become lighter — the snowshoes are simply far wider, so your weight is spread over much more snow and each patch of it is squashed far less. That squashing is what scientists call pressure. The same force over a larger area gives less pressure. Camels have broad feet for the same reason, and tractors run on fat tyres.
Or concentrate it instead
Now do the opposite. Push a drawing pin into a board. Your thumb is perfectly comfortable; the board gets a hole. It is one push, acting at both ends at the same time. The head is wide, so the pressure on your thumb is low. The point is tiny, so the pressure on the board is enormous. Knives, needles, nails and teeth all work on this principle, and sharpening something does not make it stronger — it makes the area smaller.
Curriculum links: NGSS MS-PS2-1