What is Pressure?
When you push your thumb into a piece of clay, the force you apply spreads over the area of your thumb. The pressure is what determines how much the clay deforms — not just the force alone. This is why a drawing pin pressed with the same force goes into a board much more easily than a flat coin: the pin has a much smaller area.
Pressure is the force applied per unit area of a surface. It measures how concentrated a force is over a given area.
The key relationship is:
Where:
- = pressure (pascals, Pa)
- = force applied (newtons, N)
- = area over which force acts (m²)
The SI unit of pressure. 1 pascal = 1 newton per square metre (1 Pa = 1 N/m²). Named after French scientist Blaise Pascal.
Think about walking on snow. If you wear normal shoes, your weight is concentrated on a small area and you sink right in. Snowshoes spread your weight over a much larger area, reducing the pressure and letting you glide across the surface. Same force — very different pressure!
Calculating Pressure: Worked Examples
Example 1: A person standing on the ground
A person has a weight of 600 N. The total area of their feet in contact with the ground is 0.03 m².
Find the pressure they exert on the ground.
Step 1: Write the formula:
Step 2: Substitute the values:
Step 3: Calculate:
The person exerts a pressure of 20,000 Pa on the ground.
Example 2: A sharp knife vs. a blunt knife
A chef applies a force of 30 N with a sharp knife (blade contact area = 0.000 003 m²) and the same force with a blunt knife (contact area = 0.000 03 m²).
- Sharp knife: Pa = 10 MPa
- Blunt knife: Pa = 1 MPa
The sharp knife exerts 10× more pressure — this is why sharp knives cut more effectively!
Always check your area units. Pressure calculations often fail because areas are left in cm² instead of m². Remember: 1 cm² = 0.0001 m² (divide cm² by 10,000 to get m²).
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