2.7 - Pressure Equation & How It Works
Pressure is force per unit area
- Pressure tells you how concentrated a force is over the area it acts on.
- The same force over a small area gives a high pressure; over a large area it gives a low pressure.
- For example, a sharp knife concentrates force onto a tiny area to produce high pressure, while snowshoes spread weight over a large area to produce low pressure.
Fluid pressure
- Gases and liquids are both fluids - their particles are free to move around and flow.
- As particles move, they collide with surfaces and exert a force on them, creating pressure.
- The force from fluid pressure always acts normal (at right angles) to the surface.
Worked example - Pressure from force and area
A box rests on the floor. It presses down with a force of 200 N over a contact area of 0.5 m2. Calculate the pressure the box exerts on the floor.
Step 1 - Formula
Step 2 - Substitution and calculation
The box exerts a pressure of 400 Pa on the floor.
Formula for pressure
Where:
- = Pressure
- = Force acting normal to the surface, in newtons (N)
- = Area of that surface
The pressure on a surface is found from the force acting normal to the surface and the area of that surface.
Cover the quantity you want to find to get the equation to use.
Keeping units consistent
- When the area is in m2, pressure is in pascals (Pa), where 1 Pa = 1 N/m2.
- When the area is in cm2, pressure is in N/cm2.
- When the area is in mm2, pressure is in N/mm2.
- The same equation works for solids, liquids and gases.
TermDefinition
PressureThe force acting at right angles to a surface per unit area of that surface.
FluidA liquid or a gas, whose particles can move past one another so it can flow.
NormalA line drawn at right angles to a surface or boundary at the point where a ray or wave meets it. A force or pressure that acts at right angles to a surface is also said to act normal to it.
PascalA unit of pressure equal to one newton per square metre.