Pressure · Grade 8
Pressure tells you how concentrated a force is: the same force on a smaller area gives a bigger pressure. That is why knives are sharp and snowshoes are wide. In liquids and gases pressure acts in all directions and increases with depth, which is why dams are thicker at the bottom.
Key facts
- Pressure = force ÷ area: p = F/A. 1 Pa = 1 N/m².
- Pressure in a liquid: p = ρgh (density × g × depth).
- Total pressure underwater = atmospheric pressure (about 101 kPa) + ρgh.
- Hydraulics: pressure is passed on equally, so (F₁)/(A₁) = (F₂)/(A₂).
- Atmospheric pressure decreases with height because there is less air above.
- Flow: the volume per second is the same through a pipe, so A₁v₁ = A₂v₂.
Worked examples
Example 1
A 600 N person stands on snowshoes with a total area of 0.30 m². Find the pressure on the snow.
Answer: 2,000 Pa
Example 2
Find the total pressure on a diver 20 m deep in sea water (ρ = 1,030 kg/m³). Atmospheric pressure is 101 kPa. (g = 9.8 N/kg)
Answer: 302.9 kPa