Pressure

This lesson covers: 

  1. What pressure is and how it is calculated
  2. Atmospheric pressure
  3. Pressure in liquids

What is pressure?

Pressure is defined as the force applied over a certain area.

Calculating pressure

Diagram showing the formula triangle for calculating pressure with force and area.

Pressure can be calculated using the formula:

Pressure = AreaForce​


The formula triangle shows the relationship between pressure, force and area.

Calculating pressure example

Illustration showing a 5 N force applied over an area of 2 square metres.

For example:

A force of 5 newtons (N) is applied over an area of 2 square metres (m2).

Calculate the pressure.


Step 1:

Use the formula: Pressure = AreaForce​

Step 2:

Substitute the correct values:Pressure = 25​

Step 3:

Complete the calculation: Pressure = 25​= 2.5

Pressure = 2.5 N/m2

Pressure is measured in pascals

Pressure is measured in units of newtons per square metre, which is given the name pascals (Pa).

  • 1 pascal = 1 N/m2
  • Applying 1 N of force over 1 m2 results in a pressure of 1 Pa.

Atmospheric pressure

The atmosphere surrounding Earth exerts a downward force due to its weight. This creates atmospheric pressure.

Diagram showing atmospheric pressure differences between high and low elevation, with fewer air particles at high elevation and more air particles at low elevation.
  • Pressure increases closer to sea level since there is more atmosphere weighing down from above.
  • Pressure decreases with height/elevation since there is less atmosphere above weighing down.
  • Pressure exceeds 100,000 Pa at sea level but drops to ~33,000 Pa at the top of Mt. Everest (~8800 m elevation).

Pressure in liquids

Illustration showing an object floating when weight equals upthrust and sinking when weight is greater than upthrust.

In liquids like water:

  • Pressure increases with depth, due to the greater weight of liquid pressing down from above.
  • Pressure causes upthrust - an upward force which acts on the object due to being in a liquid.


Will it sink or float?

An object will float if the weight is equal to the upthrust.

An object will sink if the weight is greater than the upthrust.