Pressure in Solids, Liquids and Gases: KS3 Physics Guide

Pressure is the force acting on a unit area of a surface. The same force spread over a larger area creates a lower pressure; concentrated onto a smaller area, it creates a higher pressure. This concept explains everything from why a drawing pin pierces paper to why submarines need thick hulls — and it is a key part of KS3 physics, covered in Year 8 or Year 9.

What is pressure and how is it calculated?

Pressure is defined as the force applied per unit area, measured at right angles to the surface.

The formula is:

pressure = force ÷ area

Written using symbols:

P = F ÷ A

Where:

P = pressure, measured in pascals (Pa) or newtons per square metre (N/m²) — these are identical (1 Pa = 1 N/m²)

F = force, measured in newtons (N)

A = contact area, measured in square metres (m²)

Rearranging the formula

To find force: F = P × A

To find area: A = F ÷ P

Worked example: pressure from a solid

Question: A crate weighs 600 N. Its base measures 1.5 m × 0.4 m. What pressure does the crate exert on the floor?

Working:

Area = 1.5 × 0.4 = 0.6 m² P = F ÷ A = 600 ÷ 0.6 = 1000 Pa

Everyday comparisons: Atmospheric pressure at sea level is approximately 101 325 Pa (around 100 000 Pa, or 100 kPa). A car tyre is typically inflated to around 220 000–250 000 Pa (220–250 kPa) above atmospheric pressure.

How does area affect pressure?

Area and pressure are inversely proportional (when force is constant): doubling the area halves the pressure, halving the area doubles the pressure.

Object | Effect of design | Reason
Drawing pin / nail | Pointed tip = tiny area → very high pressure | Penetrates material easily
Snowshoes | Wide base = large area → low pressure | Walker does not sink into snow
Tractor tyres (wide) | Large contact area → low ground pressure | Reduces soil compaction
Knife blade (thin edge) | Tiny area → high pressure | Cuts food with less force
Bed of nails (many nails) | Force shared over many points → low pressure per nail | Performer does not get punctured

Pressure in liquids

Liquids exert pressure in all directions on any surface they are in contact with. Liquid pressure has three key characteristics:

Pressure increases with depth. The deeper you go, the more liquid is above you pressing down. At 10 m depth in seawater, pressure is approximately twice atmospheric pressure — this is why deep-sea creatures and submarines must withstand enormous forces.

Pressure acts in all directions. A liquid pushes not just downwards but sideways and upwards (upthrust). This is why a balloon submerged in water is squeezed from all sides, and why a cork released underwater pops upward.

Pressure depends on the density of the liquid. A denser liquid (e.g. seawater at ~1025 kg/m³) exerts greater pressure at any given depth than a less dense liquid (e.g. fresh water at 1000 kg/m³).

Why holes in the side of a bottle show pressure differences

If you make three holes in the side of a full water bottle — one near the top, one in the middle, one near the bottom — water shoots furthest from the bottom hole. The lowest hole has the greatest depth of water above it, so the pressure there is greatest, giving the water more force to push outward.

Pressure in gases

Gases also exert pressure. Gas particles move rapidly and randomly, colliding with the walls of their container. Each collision exerts a tiny force; the total effect of billions of collisions per second gives gas pressure.

Key points at KS3:

Atmospheric pressure is caused by the weight of the column of air above us. At sea level, this is approximately 101 325 Pa. At the top of a mountain, there is less air above, so atmospheric pressure is lower — this is why breathing feels harder at altitude.

Increasing temperature increases gas pressure (if volume is constant) because particles move faster and collide with container walls more often and with more force.

Decreasing volume increases pressure (if temperature is constant) because the same number of particles collides with a smaller area more frequently. This is the principle used in bicycle pumps and syringes.

How do we use pressure practically?

Application | How pressure is used
Hydraulic brakes (cars) | Pressure applied to brake fluid (liquid) is transmitted equally in all directions; small force on pedal → large force on brake pads
Drinking through a straw | Sucking reduces air pressure inside the straw; atmospheric pressure on the liquid surface pushes liquid up the straw
Weather forecasting | Barometers measure atmospheric pressure; low pressure often predicts rain; high pressure often predicts dry weather
Scuba diving | Divers breathe from tanks at elevated pressure to match the water pressure at depth, allowing lungs to function normally
Syringes / vacuum pumps | Pulling back the plunger reduces pressure inside, allowing atmospheric pressure to push liquid in

The Department for Education's Science Programmes of Study for Key Stage 3 requires pupils to calculate pressure using the formula, understand how pressure varies with depth in liquids, and relate gas pressure to the movement of particles. BBC Bitesize KS3 Physics covers pressure calculations, liquid and gas pressure, and atmospheric pressure with worked examples and test questions.

Frequently asked questions

What is the difference between force and pressure?

Force and pressure are related but distinct. Force is the push or pull on an object, measured in newtons (N). Pressure is the force concentrated onto a unit of area, measured in pascals (Pa = N/m²). The same force can produce very different pressures depending on the area it acts over: a force of 100 N spread across a 1 m² surface gives 100 Pa, but concentrated onto 0.01 m² (the area of a stiletto heel) it gives 10 000 Pa — 100 times greater. This is why high heels can dent soft flooring that flat shoes do not.

Why does pressure in a liquid increase with depth?

Pressure in a liquid increases with depth because of the weight of liquid above. At any depth, the liquid below has to support the weight of all the liquid on top of it, plus the weight of the atmosphere above the liquid surface. The deeper you go, the more mass of liquid is above you, and therefore the greater the downward force per unit area — the greater the pressure. Divers experience this as increasing ear pain with depth (the eardrum is pushed inward by the higher pressure), and deep-sea fish have very flexible or absent swim bladders to avoid being crushed.

Why does atmospheric pressure decrease with altitude?

Atmospheric pressure is caused by the weight of air above a point. At sea level, you have the entire thickness of the atmosphere (roughly 100 km) pressing down. As you climb a mountain or rise in an aircraft, the amount of air above you decreases, so the atmospheric pressure drops. At the summit of Mount Everest (8 849 m), atmospheric pressure is only about 34 kPa — roughly one-third of sea-level pressure. There is still enough oxygen, but each breath contains far fewer oxygen molecules, which is why climbers often use supplementary oxygen above about 8 000 m.

How do hydraulic systems use pressure?

Hydraulic systems exploit the fact that pressure is transmitted equally throughout an enclosed liquid (Pascal's law). A small force applied to a small piston creates a pressure in the fluid. That same pressure acts on a much larger piston, producing a proportionally larger force. In a car's braking system, a modest force on the brake pedal pressurises brake fluid in narrow pipes; that pressure acts on the larger area of the brake caliper pads, producing a much larger force to clamp the disc. Hydraulic systems are also used in diggers, cranes, aircraft landing gear, and dentist's chairs.

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