3. Pressure
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An equation & units for pressure P.7 of 13 

What is pressure?
Pressure is how much force there is on a given area. So, whilst an elephant weighs more than a horse, the horse pushes down with a bigger pressure. This is because the areas of their feet are different.

Pressure depends on two variables: force and area (see page 6).

They make an equation as follows:

An equation for pressure
Equation
Picture 3.2 An equation for pressure.
Picture 3.2 shows the equation for pressure. The force is on the top of the fraction. Therefore a bigger force will produce a bigger pressure - which is what we expected. Area is on the bottom of the fraction so a bigger area will produce a smaller pressure.
What are the units?
We usually measure force in newtons (N) and area in square centimetres (cm2). The unit for pressure will then be N/cm2. Notice that we get the new unit by putting the old units into the equation (newtons ÷ centimetres squared = N/cm2).
The pascal
The pascal (Pa) is a standard unit for pressure. One pascal is equivalent to 1 N/m2. So, to get the pressure in pascals, you need to measure the force in newtons and the area in square metres (rather than square centimetres).

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Question 7
Look at the examples below. In each case work out the pressure.
Description Pressure
A. A woman weighs 600 N and the total area of her high heels is 20 cm2. N/cm2
B. An elephant weighs 12,000 N and the total area of its feet is 1,500 cm2. N/cm2
C. A digger has a weight of 40,000 N. The area of its tracks is 8,000 cm2. N/cm2
D. An arctic explorer weighs 600 N. She wears snow shoes with an area of 1200 cm2. N/cm2
E. Which one will sink into the ground:
            i)  the most
            ii) the least

Summary                                           Close
  • pressure is the force per unit area
  • we measure pressure in pascals (Pa)
  • pressure = force ÷ area
  • 1 Pa = 1 N/m2