1. Junior Certificate Science
Pressure

2. Forces We have already seen that forces can:
Cause and object to changes it velocity or shape or,
If an objects is fixed forces can make it rotate around a fixed point (called a fulcrum).
We now look at the link between forces and area.
Which would hurt more?
If the spire fell on you at the bottom or
If it fell on you at the top?

4. Pressure Pressure is the force acting on a unit of area.
A force acting on a small area is much more effective that a large one.
This is why we use sharp objects to cut through something as the force is concentrated on a smaller area.
If the area is bigger the effect of the force is less.
Example, a heavy digger uses tracks to spread the force over a large area.
Can you think of other examples of where we use this phenomenon designing things?

6. Calculating Pressure
To calculate pressure we use the following formula, which you must remember!!
Pressure = Force ÷ Area
The unit of forces is the Newton and the unit of area is m2 or cm2.
So the unit of pressure is N/ m2 or N cm2.
But the standard unit of pressure, N/ m2, is called the Pascal.
We can rewrite this formula to show that
Force = Pressure x Area
Area = Pressure x Force

7. Example 1
A block of mass 5kg has a base of 5cm and width 2 cm. What is the pressure on the base.
Pressure = Force ÷ Area
But I don’t have the force yet.
Force = Mass x 10 (5kg x 10 = 50N)
Also Area = Length x Width (5cm x 2cm = 10cm2)
So Pressure = 50N ÷ 10cm2 = 5 N cm2.
Answer = 5 N cm2

8. Pressure in Liquids and Gases
Pressure is a useful quantity to use when talking about forces exerted by liquids and gases since these act over an area.
There is one difference between the pressure exerted by a solid block and the pressure exerted by a liquid or gas.
The liquid or gas exert pressure in all directions, but is greatest at the bottom.
Example, a scuba diver is feeling pressure from all over his body, not just the top.
The air pressure is lower on top of a mountain than at sea level.

10. Testing Water Pressure
Get a plastic bag and fill with water.
Make holes all over the bag.
Now fill the bag with water.
What happens?
Well all the holes will start leaking water, but the water will some out faster at the bottom.
This shows that area is spread out all over the liquid but greatest at the bottom.

11. Pressure and Depth
We can explain why the pressure is greater at the bottom.
At the bottom of the bag there is more liquid, more liquid means more force acting on its mass.
As scuba divers go deeper the weight of water increases and the diver feels much higher pressure.
To go some depths, special diving equipment is required to deal with the high pressures.
In a dam, the walls at the bottom are thicker because the pressure is much larger.

14. Water Supply and Pressure
Water will always flow so that it is all at the same level.
This fact is used in supplying water to businesses and houses.
Reservoirs are usually have a greater height than the houses it supplies.
When the water is releases from the reservoir it tries to find the same level as the water in the house.
This pressure that causes the water to be pushed into the tank in the attic of the house.
This works very well, provided your house is below the level of the reservoir.

15. Everyday Applications of Pressure
The relationship between force and pressure provides a means of applying large forces using liquids.
In the piston of a car engine, a force is applied to a small area but transferred to a larger area.
The pressure in liquids transferred to pistons is used in: Brake systems in lorries, lifting gear in forklifts, lifting buckets in JCB’s and moving arms of industrial robots.

18. The atmosphere and its pressure.
Our planet is covered in a layer of gases.
Gas is matter and all matter has mass.
So gravity pushes down on the air in our atmosphere causing a pressure.
This pressure is called Atmospheric Pressure.
It is estimated that the total mass of air is around 500 million million tonnes!!!
Around 1 tonne of air is exerting a pressure on your shoulders!
We don’t feel this because air inside us is exerting an equal pressure back!

19. The Crushing Can Experiment
We can show how the atmosphere has pressure by removing the air from something.
When you take the air out of something, the pressure of the air in the atmosphere exerts a pressure but there is no equal pressure to exert back.
The object is then crushed.
The Can Crushing demonstration shows that the atmosphere has pressure!

21. Measuring Atmospheric Pressure
Atmospheric Pressure is not constant.
It changes according to temperature and moisture in the atmosphere.
We use a Barometer to measure pressure.
One type of barometer we use is called a mercury barometer.
We measure atmospheric pressure based on the column of liquid mercury that can be supported by atmospheric pressure alone.

22. Mercury Barometer
As the pressure in the atmosphere increases the mercury is pushed further up the column.
The average height of a column of mercury is 76cm.
This is equal to 1013 hPa.
If you watch a weather forecast you see the atmospheric pressure being displayed.

24. The Altimeter One use of barometers is in aircraft.
As an aeroplane climbs up through the atmosphere the pressure drops
This is because there is less air above it pushing down.
This drop in pressure can be measured and translated into altitude.

26. Atmospheric Pressure and Weather
The atmospheric pressure is a major influence on the weather.
Warm, moist air exerts less pressure than cold dry air.
So when the atmospheric pressure drops you can expect rain and winds.
When the pressure is high you can expect warm dry weather.
On a map, areas with the same atmospheric pressure are drawn using lines called isobars.