Section 1 of 6What Is Pressure?
The same force can have very different effects depending on the area it acts on.
Definition
- Pressure is the force acting normally on each unit of area of a surface.
Equation
- Pressure = force ÷ area, written p = F ÷ A.
Units
- Pressure is measured in pascals (Pa), where 1 Pa = 1 N/m².
Area
- The area is the cross-sectional area of contact, so the force acts at right angles to the surface.
Section 2 of 6Changing the Pressure
Changing the Pressure
Larger area
- Spreading a force over a bigger area reduces the pressure, which is why snowshoes stop people sinking.
Smaller area
- Concentrating a force on a small area increases the pressure, as with a sharp knife or a drawing pin.
Larger force
- For the same area, a bigger force produces a bigger pressure.
Rearranging
- F = p × A and A = F ÷ p, both of which follow from the pressure equation.
| Quantity | Symbol | Unit |
|---|---|---|
| Pressure | p | Pa (N/m²) |
| Force | F | N |
| Area | A | m² |
Section 3 of 6Pressure in Gases
Pressure in Gases
Particle collisions
- Gas particles move randomly and collide with the walls of their container.
Force
- Each collision exerts a tiny force on the wall, and the many collisions together create a steady force.
Pressure
- This force acts over the area of the wall, so the gas exerts a pressure.
Direction
- The pressure acts at right angles to the surface at every point.
Section 4 of 6Solving a Pressure Problem
1
List the known quantities and check their units.
2
Convert any areas in cm² to m² by dividing by 10 000.
3
Write the equation p = F ÷ A, rearranging if required.
4
Substitute the values into the equation and calculate the answer.
5
Give the answer in pascals and check that the size is sensible.
Section 5 of 6Worked Example
Question
- A person of weight 600 N stands with both feet on the ground, with total contact area 0.040 m².
- Calculate the pressure, then the pressure if the same person stands on one stiletto heel of area 0.00050 m².
- Pressure on both feetp = F ÷ A = 600 ÷ 0.040 = 15 000 Pa.What the symbols mean: p = pressure (Pa = N m−2) · F = force acting at right angles to the surface (N) · A = area (m2)
- Pressure on the heelp = 600 ÷ 0.00050 = 1 200 000 Pa.
- The heel pressure is 80 times larger because the area is 80 times smaller (0.040 ÷ 0.00050).
Answer 15 000 Pa (15 kPa) on both feet and 1 200 000 Pa (1.2 MPa) on the heel.
Section 6 of 6Exam Checklist & Tips
1.8.1 – Key Points for the Exam
- Pressure Force acting per unit area.
- Pascal The unit of pressure; 1 Pa = 1 N/m².
- Newton The unit of force.
- Collision An impact between a gas particle and a surface that produces a force.
Common Mistakes
- Using an area in cm² without converting to m² before calculating.
- Using the weight in kg rather than the force in N.
- Saying pressure and force are the same quantity.
- Forgetting that gas pressure acts in every direction on the walls.