4PH1

Density & Pressure

Solids, Liquids & Gases

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 8% of your exam marks.

stable

Low

Stable8%

ρ = m/V and pressure calculations (P = F/A, P = hρg) appear as standard calculation questions.

Pressure is the force per unit area acting normal (at right angles) to a surface
The same force concentrated into a smaller area produces a higher pressure. That's why a sharp knife slices and a blunt one doesn't, and why a sleeping bag is more comfortable on a wide foam mat than on a single rope

P = F / A

where:
- P = pressure (pascals, Pa, where 1 Pa = 1 N/m²)
- F = force acting perpendicular to the surface (N)
- A = area over which the force is applied (m²)
Rearrangements:
- F = P × A (the total force a fluid pressure exerts on a surface of known area)
- A = F / P

Worked example

Calculate pressure from force and area

A person weighing 600 N stands on a flat tile with a total contact area of 0.030 m².

Solution:

Small area + ordinary force = high pressure:
- Drawing pins taper to a tiny tip on the wall side. A modest push from a fingertip becomes a huge pressure at the point, driving it into the wood
- Nails and needles work the same way
- Sharp knives cut better than blunt ones because the cutting edge is a much smaller area
- Stiletto heels focus the wearer's weight onto a few square millimetres and can dent wooden floors
Large area + ordinary force = low pressure:
- Tractor tyres are very wide. The same vehicle weight spread over more rubber gives a lower ground pressure, so the tractor doesn't sink into wet soil
- Caterpillar tracks on tanks and bulldozers do the same thing, distributing the weight over a long track footprint
- Snow shoes and skis stop a person sinking into soft snow by spreading their weight across a much larger area than a pair of boots would
- A camel's foot broadens when it pushes down, spreading the camel's weight on sand