Pressure
Pressure is defined as the amount of force acting per unit area. The S.I. unit of pressure is newton per square metres (N/m2) or Pascal (Pa).
Pressure depends on the amount of force used and the area of contact. A force acting on a small area of contact produces a greater pressure than the same force acting on a larger area.
Take this example. Nathan has two pencils. One is very sharp. The other is very blunt. He pushed the sharp pencil and the blunt pencil into an eraser. He took care to use the same amount of force both times. He noticed that, compared to the blunt pencil, the sharp pencil went deeper into the eraser. Because it has a sharp tip, a smaller surface area comes into contact with the eraser. As the surface area is smaller in the case of the very sharp pencil, the pressure is greater. This allows the pencil to go deeper. Likewise, because the blunt pencil has a tip with a bigger surface area, it exerts less pressure on the eraser and thus does not poke in as deeply. Now that this is clear, let us see how to calculate pressure.
Calculating Pressure
Pressure is calculated by the following formula:
Force is measured in newtons and area is measured in square metres. Thus, pressure is measured in newtons per square metres (N/m2)
Let us look at the following examples of how to calculate pressure.
Example 1
An elephant and a lady stand on a floor. The lady’s shoes have sharp heels.
Which will damage the floor more – the elephant? Or the heel of the lady’s shoe? The elephant exerts a greater force, as it is heavier. But the area of the lady’s heel is less than the elephant’s foot.
Shall we answer our question by doing some calculations? Here goes.
Elephant:
Take the weight of the elephant as 40 000 N. Imagine the area of the floor as 1 000 cm2.
Lady:
Take the weight of the lady as 400 N and the area of the shoe heel as 1 cm2
So now what is the answer to our question? It is clear from our calculations. The pressure by the shoe heel is ten times greater. The sharp heel will damage the floor more than the elephant will.
Example 2
Ian took a wooden block with a weight of 2 000 N and dimensions of 4m × 1m × 0.2 m. What is the pressure on the ground when it is laid:
(a) On its largest surface?
(b) On its smallest surface?
(a) Area of the largest surface = 4m × 1m = 4 m2
Area of the smallest surface = 0.2 m × 1m = 0.2 m2
Application of Pressure
Here are some examples of how we use pressure.
The studs on football boots decrease the area of contact with the ground. This increases the pressure, causing the studs to easily sink into the ground, thus preventing the player from slipping.
A bulldozer uses steel tracks. These steel tracks have a large surface area. We all know that bulldozer is heavy. However, its weight is spread over a large aera. So, the pressure on the ground is not significant. Therefore, the bulldozer does not sink easily into mud or sand.
Fun Facts About Pressure
- Did you know that as pressure increases, the boiling point of a liquid also increases? Water boils at a lower temperature at higher altitudes. Why? Because of lower atmospheric pressure.
- The force exerted by blood against the walls of blood vessels is called blood pressure. It is commonly known as BP. Your health is optimal only if your blood pressure is normal.
- There is intense pressure deep within the Earth’s mantle. Also very high temperature. In these conditions, carbon atoms bond and form diamonds.
- Submarines are designed to withstand high pressures when they are deep underwater.
- Air pressure is measured with a device named a barometer.