Lesson focus: Pressure on a solid surface depends on normal forceThe push from a surface on an object resting on it, at right angles to the surface. and contact areaThe area where two surfaces touch, such as the bottom of a shoe on the floor..
Calculate p = F/A, convert squared units correctly and explain design applications.
Build on earlier learning
Pressure in liquidsThe same force-per-area definition now applies to solid contact.
You will be successful when you can…
- State and rearrange p = F/A.
- Use N and m².
- Convert cm² correctly.
- Explain area effects.
Words that carry the science
- contact area
- Area through which surfaces transmit force.
- normal force
- Force perpendicular to a surface.
- pascal
- One newton per square metre.
- square metre
- Area of a one-metre by one-metre square.
- inverse relationship
- One quantity falls as another rises under controlled conditions.
Pressure divides force across area
01Pressure divides force across area
Pressure = normal force ÷ contact area and 1 Pa = 1 N/m².
For fixed force, smaller area gives greater pressure. Blades concentrate force; snowshoes spread it.
02Area conversions are squared
Because 1 m = 100 cm, 1 m² = 10,000 cm². Divide cm² by 10,000 to obtain m².
Convert before substituting. A calculator cannot detect a physically wrong unit conversion.
03Models need assumptions
The simple calculation assumes the force is normal and spread across the quoted area.
Soft surfaces and uneven tread change effective contact areaThe area where two surfaces touch, such as the bottom of a shoe on the floor., so real values can be estimates.
Pressure on solids
Pressure = force ÷ area. The same weight spread over a bigger area gives a smaller pressure. That is why snowshoes stop you sinking into snow, and stiletto heels dent floors.
Controls
Areas are approximate; sinking depths are illustrative. 1 N/cm² = 10 000 N/m² = 10 kPa.
Pressure beneath a block
A 600 N block rests on 300 cm².
- p = F/A.
- 300 cm² ÷ 10,000 = 0.030 m².
- p = 600 ÷ 0.030.
- p = 20,000 Pa.
How to make the reasoning visible: The pressure is 20 kPa.
Compare contact pressures
Calculate before explaining design.
| Case | Force / N | Area / m² | Pressure / Pa |
|---|---|---|---|
| Snowshoe | 700 | 0.140 | ? |
| Boot heel | 350 | 0.0007 | ? |
| Tank track | 80,000 | 8 | ? |
| Pin tip | 20 | 0.0000005 | ? |
Calculate pressures.
Explain one design.
Check your understanding
Answer from memory, then check against the suggested answer.
State the equation.
Define one pascal.
How many cm² in 1 m²?
What happens when a brick rests on its smallest face?
Why might a tank exert more pressure than a person despite wide tracks?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A crate weighs 1800 N. Its base measures 60 cm by 50 cm.
Calculate the pressure the crate exerts on the floor, in Pa. Show your working.
(5)Total for this question: 5 marks
Command word: Convert lengths before multiplying to find area.