Section 1 of 5Pressure on Solid Surfaces
Pressure measures how concentrated a force is on a surface.
Definition
- Pressure is the force acting at right angles to a surface divided by the area over which it acts.
Equation
- p = F ÷ A, with pressure in pascals, force in newtons and area in square metres.
Small area
- A force on a small area gives a large pressure, which is why nails and knives have sharp points and edges.
Large area
- Spreading the same force over a larger area lowers the pressure, as with snowshoes and the wide tyres of tractors.
Section 2 of 5Pressure in Fluids
Pressure in Fluids
Normal force
- A fluid presses on a surface with a force that acts at right angles to that surface.
All directions
- At any point in a fluid the pressure acts equally in every direction.
With depth
- Liquid pressure increases with depth and density, given by p = hρg.
Added together
- An object in a liquid feels the pressure of the liquid plus the atmospheric pressure above it.
| Situation | Area | Pressure for the same force |
|---|---|---|
| Drawing pin point | Very small | Very high |
| Drawing pin head | Larger | Low |
| Snowshoe | Large | Low, so you sink less |
Section 3 of 5Using the Equation
Compare the pressure under a person's feet with and without snowshoes.
1
A person of weight 600 N stands on snow, and the force on the snow equals the weight, 600 N.
2
With boots of total area 0.040 m², the pressure is 600 ÷ 0.040 = 15 000 Pa.
3
With snowshoes of total area 0.20 m², the pressure is 600 ÷ 0.20 = 3000 Pa.
4
The area is five times larger, so the pressure is five times smaller, and the person sinks less.
5
The same reasoning explains why a sharp knife cuts easily: the force is concentrated on a tiny area of edge.
Section 4 of 5Worked Example
Question
- A person weighing 600 N stands on snow wearing boots with a total contact area of 0.040 m².
- Calculate the pressure on the snow and compare it with the pressure when wearing snowshoes of total area 0.20 m².
- Write the equationp = F ÷ A.What the symbols mean: p = pressure (Pa = N m−2) · F = force acting at right angles to the surface (N) · A = area (m2)
- With bootsp = 600 ÷ 0.040 = 15 000 Pa.
- With snowshoesp = 600 ÷ 0.20 = 3000 Pa.
- Comparethe area is 5 times larger, so the pressure is 5 times smaller.
Answer 15 000 Pa with boots and 3000 Pa with snowshoes
Section 5 of 5Exam Checklist & Tips
2.6.5 – Key Points for the Exam
- Pressure Force per unit area acting at right angles to a surface, in pascals.
- Pascal The unit of pressure, equal to 1 newton per square metre.
- Contact area The area of a surface over which a force acts.
- Normal At right angles to a surface.
Common Mistakes
- Using the weight of the object as pressure; pressure needs the force divided by the area.
- Mixing cm² and m² in a calculation, which gives an answer 10 000 times too large or small.
- Saying a sharp object exerts a bigger force; the force is the same, but the pressure is larger.
- Forgetting that fluid pressure acts at right angles to any surface it meets.