Pressure
Stand a brick on each face and watch the same weight dig deeper when it presses on a smaller area.
1Wonder
Push your thumb on the flat head of a pin and nothing happens. Push just as hard on the point and it goes straight in. Why does a tiny point hurt so much, and why do snowshoes stop you sinking into snow?
2Explore
This activity needs a bigger screen
Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.
- Stand the brick on each face in turn. Its weight never changes. What happens to the dent?
- Look at the footprint. How much force does each square carry on each face?
- Add weights to the brick lying flat. Can you make a dent as deep as the brick on its end with no weights?
- Which face would you choose to build on soft ground? Why?
3Explain
Same weight, different dent
In boots, your whole weight presses on two small soles, so you sink into the snow. On snowshoes, the same weight is spread over a much bigger area, so you stay on top.
Pressure = force ÷ area. The same force on a smaller area gives more pressure. Spread a force out to lower the pressure; focus it on a point to raise it.
4Practice
Pressure equals…
5Recap
- Pressure = force ÷ area (for example in N/cm²).
- Smaller area → bigger pressure: pins, knives and heels.
- Bigger area → smaller pressure: snowshoes, wide tires, beds of nails.
- In a liquid, pressure grows the deeper you go.