# What is Pressure?

Show/Hide Sub-topics (Pressure | O Level)

You can push a drawing pin into a piece of wood – but you cannot push your finger into the wood even if you exert a larger force. The difference in each case is the different area in contact. The pin has a much smaller area in contact than the finger. Hence, exerts a much higher pressure that makes the substance yield to the force.

Pressure (p) is defined as the perpendicular force (F) acting on unit area of a surface or the force per unit area (A).

$$P = \frac{F}{A}$$

SI unit of pressure is the pascal or newton per metre square. (Pa or $N m^{-2}$) It is a scalar quantity.

## Example

### Find the pressure exerted on a person by a few plastic blocks of total mass 42 kg, the bottom of which in contact with the person has an area of 840 $cm^{2}$. State what will happen to the pressure if the area is now decreased to a very small value.

\begin{aligned} \text{Pressure}, \, p &= \frac{F}{A} \\ &= \frac{W}{A} \\ &= \frac{mg}{A} \\ &= \frac{42 \times 10}{840 \times 10^{-4}} \\ &= 5.0 \times 10^{3} \text{ Pa} \end{aligned}

In order to find out what will happen to the pressure with a small area, you can replace the area, A in the above equations with a value that is smaller than $840 \text{ cm}^{2}$. You will find that:

If the area is decreased to a very small value, the pressure would increase to a very large value.