Pressure, how it grows with depth, and what the atmosphere does to you.
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Thrust is the force acting perpendicular to a surface. Pressure = thrust ÷ area, unit pascal (Pa) = N/m². The same force gives greater pressure on a smaller area: a sharp knife cuts better than a blunt one, and a wide tractor tyre sinks less than a narrow one.
A force of 200 N acts on an area of 0.5 m². Find the pressure.
P = 200 ÷ 0.5 = 400 Pa.
Why do camels have broad feet?
A large area reduces the pressure on sand so they do not sink.
Why are nails pointed?
A small area gives high pressure so the nail enters the wood easily.
Pressure in a liquid acts in all directions, is the same at the same depth, and increases with depth: P = hρg, where ρ is density. Pascal's law: pressure applied to an enclosed liquid is passed on equally in all directions; hydraulic presses and brakes use this. Dams are built thicker at the bottom because pressure grows with depth.
Find the pressure due to water at a depth of 5 m (ρ = 1000 kg/m³, g = 10 m/s²).
P = 5 × 1000 × 10 = 50,000 Pa.
Why is the wall of a dam thicker at the bottom?
The pressure of water is greatest at the bottom.
State Pascal's law.
Pressure applied to an enclosed liquid is transmitted equally in all directions.
Air has weight, so the atmosphere presses on everything: atmospheric pressure at sea level is about 1.01 × 10⁵ Pa (760 mm of mercury = 1 atm). A barometer measures it: mercury in an inverted tube stands about 76 cm above the pool. Pressure falls with height, which is why climbers feel breathless and a straw works by lowering the pressure inside it.
Estimate atmospheric pressure from a mercury column of 0.76 m (ρ = 13600 kg/m³, g = 9.8).
P = 0.76 × 13600 × 9.8 = 101,293 Pa, about 1.01 × 10⁵ Pa.
Why does the mercury column not change if the tube is tilted?
Only the vertical height matters.
Why does a suction pump or a straw work?
It lowers the pressure inside and the higher outside air pressure pushes the liquid up.
1. Define pressure and its SI unit.
2. Find the pressure when 200 N acts on 0.5 m².
3. Why does a sharp knife cut better?
4. Pressure at 10 m depth in water (ρ = 1000, g = 10)?
5. State two properties of pressure in a liquid.
6. State Pascal's law.
7. Name the instrument for measuring atmospheric pressure.
8. Give the value of normal atmospheric pressure in mm of mercury.
9. Why does a diver feel more pressure deeper down?
10. Why does pressure decrease with height in the atmosphere?