Force & Pressure

Watch first

FreeScienceLessons · AQA PhysicsPressure in Fluids

Key facts

Pressure = Force ÷ Area (P = F/A). Units: Pa (pascals) or N/m².
Smaller area → greater pressure for same force. (e.g. nail vs flat surface, snowshoes)
Pressure in a liquid: P = hρg (depth × density × gravitational field strength).
Pressure in a fluid increases with depth and density.
Pressure at same depth acts equally in all directions.

Upthrust = upward force on an object in a fluid = weight of fluid displaced.
Object floats if upthrust ≥ weight.
Object sinks if weight > upthrust.
A ship floats because its average density (hull + air) is less than water.
Atmospheric pressure — caused by weight of air above. Decreases with altitude (less air above).

Exam questions — 3 questions · 8 marks

1 markPressure calculationAQA 8463 P2 style

A force of 120 N acts on an area of 0.04 m². What is the pressure?

A 4.8 Pa

B 3,000 Pa

C 4,800 Pa

D 0.033 Pa

1 markPressure with depthAQA 8463 P2 style

A diver descends deeper in the ocean. What happens to the water pressure on the diver?

A Pressure decreases as the water becomes less dense at depth

B Pressure increases because there is more water above the diver exerting its weight

C Pressure stays the same because water pressure only depends on density

D Pressure decreases because the diver moves away from the atmosphere

3 marksExplain upthrustAQA 8463 P2 style

Explain why a steel ship floats even though steel is denser than water. (3 marks)

Hint: Key: average density (steel + air) < density of water. The ship is shaped to displace enough water so upthrust = weight.

+30 XP

1 markAtmospheric pressureAQA 8463 P2 style

Why does atmospheric pressure decrease as altitude increases?

A Air is colder at higher altitudes so its pressure decreases

B There is less air above you at higher altitudes, so the weight of air pressing down is less

C Gravity decreases significantly with altitude, reducing pressure

D Atmospheric pressure is constant and does not change with altitude