I have never really written much about pressure on the blog. Perhaps it has always seemed a little dull but questions do come up about pressure so I ought to have a go. It ties in nicely with the posts about forces because pressure is force applied per square centimetre.
At C.E. pressure will always be quoted in N/cm2 but the true standard unit of pressure is the N/m2 or Pascal (Pa). You will have learned in Geography about isobars, which are regions of equal pressure on a weather map. A bar is a unit of pressure – iso just means “the same” from the Greek isos, meaning equal.
You will normally be given two out of Pressure, Force or Area and asked to calculate the missing value. The triangle might help you to remember the different formulae but it would be better to learn them.
(N/cm2) Pressure = Force ÷ Area
(cm2) Area = Force ÷ Pressure
(N) Force = Area × Pressure
If you at least remember the unit for pressure (N/cm2), then you should always be able to work out the formula for calculating any of the three properties.
There are many everyday examples where ideas about pressure play an important role. Skis increase the area of contact between you and the snow, decreasing the pressure and stopping you from sinking. When you flex your hips, you drive the edges of the skis into the snow, increasing the pressure and making them dig into the snow. Studs on rugby boots increase the pressure so that they sink into the ground to increase grip, ice skates create a very high pressure to melt the ice so that you slide along on a thin film of water. Wide tyres on tractors spread the load stopping the vehicle from sinking into soft ground, narrow blades on scissors increase the pressure so that they can cut more easily.
Here are some example questions…
[CE…] Calculate the pressure produced by a force of 600 N acting on an area of 2 cm2.
Pressure = Force ÷ Area
Pressure = 600 N ÷ 2 cm2
Pressure = 300 N/cm2
[CE…] The pressure of a gas contained in a cylinder with a movable piston is 20 N/cm2. The area of the piston is 0.5 cm2. Calculate the force that is exerted on the piston.
Force = Area × Pressure
Force = 0.5 cm2 × 20 N/cm2
Force = 10 N
[Slightly harder…] A swimming pool of width 5 m and length 20 m is filled with water to a depth of 2 m. Calculate the pressure on the bottom of the pool due to the water.
Volume of water = 5 m × 20 m × 2 m = 200 m3
Density of water = 1000 kg/m3 (or 1 g/cm3 )
Mass of water = Density × Volume
Mass of water = 1000 kg/m3 × 200 m3
Mass of water = 200,000 kg (200 tonnes)
Weight of water = Mass × Force of Gravity
Weight of water = 200,000 kg × 10 N/kg
Weight of water = 2,000,000 N
Area of pool bottom = 5 m × 20 m = 100 m2
Pressure = Force ÷ Area
Pressure = 2,000,000 N ÷ 100 m2
Pressure = 20,000 N/m2
Twig Prep has some good stuff on pressure that might be worth exploring.
Questions…
- What equation links pressure to force and area?
- How do you calculate density?
- What is the approximate value for Gravity at the Earth’s surface?
- Name a device that measures pressure.
- What happens to the pressure of a gas in a canister as the temperature inside the tank increases?
- Why is the air inside a sterile laboratory kept at a higher pressure than normal atmospheric pressure outside the laboratory?
Comments