Pressure in fluids
Pressure (Separate Physics only)
is the . You must recall this equation:
pressure = force normal to a surface ÷ area of that surface, or p = F/A
| Quantity | Symbol | Unit |
|---|---|---|
| Pressure | p | pascals (Pa) |
| Force | F | newtons (N) |
| Area | A | square metres (m²) |
Example: a 600 N student stands on feet with a total area of 0.04 m².
- p = F/A = 600 ÷ 0.04
- p = 15 000 Pa
A is a liquid or a gas. The pressure in a fluid makes a force at right angles (normal) to any surface it touches.
Snowshoes spread your weight over a large area, so the pressure is low. A sharp knife has a tiny area, so the pressure is high.
Key terms
Pressure in liquids (Higher tier only) (Separate Physics only)
The deeper you go in a liquid, the more liquid there is above you, so the of liquid pushing down is bigger and the . A denser liquid gives a bigger pressure at the same depth. This equation is on the equation sheet:
, or p = hρg
| Quantity | Symbol | Unit |
|---|---|---|
| Pressure | p | pascals (Pa) |
| Height (depth) of liquid | h | metres (m) |
| Density | ρ | kilograms per cubic metre (kg/m³) |
| Gravitational field strength | g | newtons per kilogram (N/kg) |
Example: find the pressure from the water on a diver 10 m deep in water of density 1000 kg/m³.
- p = hρg = 10 × 1000 × 9.8
- p = 98 000 Pa
This is why dams are built thicker at the bottom.
Upthrust, floating and sinking (Higher tier only) (Separate Physics only)
An object in a liquid has more liquid above its bottom surface than above its top surface. So the pressure on the bottom is greater than the pressure on the top. This makes a , called .
- An object floats when the upthrust equals its weight before it is completely under the surface. This happens if it is less dense than the liquid.
- An object sinks if it is denser than the liquid. Even when it is completely under, its weight is bigger than the upthrust.
A steel ship floats because its hollow shape holds a lot of air, so its average density is less than water's.
Key terms
Atmospheric pressure (Separate Physics only)
The atmosphere is a thin layer of air around the Earth. Air molecules collide with every surface, and these collisions cause . At sea level it is about 100 000 Pa.
The atmosphere gets less dense the higher you go. So as height increases:
- there are fewer air molecules above you, so the above you is smaller
- there are fewer collisions on each surface
So atmospheric pressure decreases with height. That's why your ears can pop in a plane or on a mountain road.