Pressure as force per area, p = F/A with areas in cm², pressure in a column of liquid p = hρg (Higher), upthrust and floating (Higher), and atmospheric pressure.
What you will learn
- Calculate pressure, force or area with p = F/A
- Convert cm² to m²
- Use p = hρg (Higher)
- Explain upthrust and floating (Higher)
- Explain why air pressure falls with height
Force spread over an area
pressure = force normal to a surface ÷ area of that surface
p = F ÷ A
p in pascals (Pa), F in N, A in m². 1 Pa = 1 N/m². All three boards: recall.
A fluid (a liquid or a gas) presses at right angles (normal) to every surface it touches.
- A 60 kg dancer on one foot, area 150 cm², g = 9.8 N/kg
- F = 60 × 9.8 = 588 N
- A = 150 ÷ 10 000 = 0.015 m²
- p = 588 ÷ 0.015 = 39 200 Pa = 39.2 kPa
1 m² = 100 cm × 100 cm = 10 000 cm². Dividing cm² by 100 is the classic slip.
Rearranged: force F = p × A, and area A = F ÷ p. Change kPa to Pa first (× 1000).
The rest of this lesson and its 9 practice questions are locked.
In this lesson
- Force spread over an area
- Deeper means more pressure (Higher)
- Upthrust and floating (Higher)
- The atmosphere
- Try it: Pressure
9 practice questions, marked as you go; about 25 minutes in all.
Key terms
Upthrust
In this unit: Forces, work, elasticity, moments and pressure
Weight and resultant forces, work done, stretching springs and their required practical, moments, levers and gears, and pressure in fluids.