Course: Separate Physics only | Tier: Foundation + Higher (liquid pressure and upthrust HT) | Linked equations: p = F/A, p = hρg | Lesson: Deep Sea Diver
Notes
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- A fluid is a liquid or a gas.
- The pressure in a fluid causes a force at right angles (normal) to any surface.
- Pressure = force ÷ area (p = F/A). Unit: pascal (Pa), where 1 Pa = 1 N/m².
- (HT) Pressure in a liquid increases with depth and with the density of the liquid: p = h × ρ × g.
- (HT) Upthrust: a submerged object has a greater pressure on its bottom surface than its top, so there is a resultant upward force.
- (HT) An object floats if its weight equals the upthrust, i.e. if it is less dense than the fluid. It sinks if it is denser than the fluid.
Atmospheric pressure
The atmosphere is a thin layer of air around the Earth that gets less dense with height. Air molecules colliding with a surface create atmospheric pressure.
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Atmospheric pressure decreases with height. As you go higher, there is less air above you, so the weight of air above a surface is smaller. There are also fewer air molecules, so there are fewer collisions with the surface.
Questions
Q1 (F) In which direction does the force from a fluid’s pressure act on a surface?
Show answer
At right angles (normal) to the surface.
Q2 (F) A force of 600 N acts on an area of 0.20 m². Calculate the pressure.
Show answer
F: p = F ÷ A
I: p = 600 ÷ 0.20
A: 3000 Pa
Q3 (F) Explain why atmospheric pressure is lower at the top of a mountain.
Show answer
There is less air above you, so the weight of air above is smaller. The air is also less dense, so fewer molecules collide with surfaces.
Q4 (HT) Calculate the pressure due to the water at a depth of 10 m. (Density of water = 1000 kg/m³, g = 9.8 N/kg)
Show answer
F: p = h ρ g
I: p = 10 × 1000 × 9.8
A: 98 000 Pa
Q5 (HT) Explain what causes upthrust on a submerged object.
Show answer
Pressure increases with depth, so the pressure on the bottom surface is greater than on the top surface. This gives a bigger upward force than downward force, so there is a resultant upward force.
Q6 (HT) Explain why a block of wood floats in water but a steel block of the same size sinks.
Show answer
Wood is less dense than water, so it sinks only until the upthrust equals its weight, then floats. Steel is denser than water, so even when fully submerged the upthrust is less than its weight, and it sinks.
Q7 (HT) A diver goes from 5 m to 20 m deep in the sea (density 1030 kg/m³, g = 9.8 N/kg). Calculate the increase in pressure due to the water.
Show answer
Change in depth = 20 − 5 = 15 m
F: Δp = h ρ g
I: Δp = 15 × 1030 × 9.8
A: 151 410 Pa (about 150 000 Pa)
