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Pressure in fluids and upthrust
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Pressure in a liquid increases with depth. This explains why things float. Separate Physics only.
Pressure
pressure = force ÷ area, p = F ÷ A. Units: pascals (Pa), where 1 Pa = 1 N/m2.
A fluid (liquid or gas) pushes at right angles on every surface it touches.
A fluid (liquid or gas) pushes at right angles on every surface it touches.
Pressure with depth
In a liquid: p = h × ρ × g: pressure (Pa) = depth (m) × density (kg/m3) × g (N/kg).
The deeper you go, the greater the weight of liquid above, so the greater the pressure.
Atmospheric pressure decreases with height, because there is less air above.
The deeper you go, the greater the weight of liquid above, so the greater the pressure.
Atmospheric pressure decreases with height, because there is less air above.
Upthrust and floating
The pressure on the bottom of an object in a fluid is greater than on the top. This gives an upward force: upthrust.
Upthrust equals the weight of fluid displaced.
An object floats if its weight equals the upthrust before it is fully submerged, which happens when it is less dense than the fluid.
Upthrust equals the weight of fluid displaced.
An object floats if its weight equals the upthrust before it is fully submerged, which happens when it is less dense than the fluid.
Find the pressure from the water 5 m below the surface. (ρ = 1000 kg/m3, g = 10 N/kg)
- p = h × ρ × g
- 5 × 1000 × 10
Answer: 50 000 Pa
p = F ÷ A. p = hρg in a liquid. Upthrust equals the weight of fluid displaced. Objects less dense than the fluid float.
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