Walk across deep snow in sneakers and you sink. Strap on snowshoes and you stay on top, though your weight is the same. What changed is how the force is spread out. Pressure is a unit rate: force per square meter.
Key idea
Pressure is force divided by the area it pushes on: $$P = \dfrac{F}{A}$$ in pascals (Pa), where $1\,\mathrm{Pa} = 1\,\mathrm{N/m^2}$.
Why it works
Each square meter of snow can hold up only so much force before it gives way. Snowshoes spread your weight over several times more area, so each square meter carries only a fraction of it. A knife works the other way: its thin edge puts the force on a tiny area, so the pressure is enormous.
Worked example: Standing in snow
A hiker weighing 600 N stands on boots with a total area of $0.04\,\mathrm{m^2}$. Then she puts on snowshoes with a total area of $0.2\,\mathrm{m^2}$. Find the pressure each time.
- Boots: $P = \dfrac{600\,\mathrm{N}}{0.04\,\mathrm{m^2}} = 15{,}000\,\mathrm{Pa}$.
- Snowshoes: $P = \dfrac{600\,\mathrm{N}}{0.2\,\mathrm{m^2}} = 3{,}000\,\mathrm{Pa}$.
- Five times the area, one fifth the pressure.
Answer: 15,000 Pa in boots; 3,000 Pa on snowshoes
Common mistake
Thinking a bigger force always means more pressure. Pressure depends on the area too: a 600 N hiker on snowshoes presses on the snow less than a 300 N child in small boots. Check: compare force ÷ area, not force alone.
The air around you presses on everything with about 101,000 Pa at sea level. You don't feel it because the pressure inside your body matches it, pushing outward just as hard.