3. Pressure
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Pressure and area P.9 of 13 

What is pressure?

On this page, you will do an imaginary experiment to find out how pressure varies with area. To keep it a fair test, the force will be the same all the way through.

Elephant
Picture 3.4 How does pressure change with the number of feet on the ground?

Look at the elephant in picture 3.4. She is pretty heavy – 12,000 N.
However, let’s see what pressure she creates. Each foot has an area of 300 cm2. When she has all four feet on the ground, what is the pressure under her feet?

Pressure =

= N/cm2
Don't type in any commas - just the numbers

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Summary                                           Close
  • pressure is the force per unit area
  • pressure gets smaller as the area gets bigger
  • we measure pressure in pascals (Pa)
  • pressure = force ÷ area
  • 1 Pa = 1 N/m2
Now she stands up on her back legs.
So she only has two feet on the ground.

What is the pressure now?

Pressure =

= N/cm2

Good. Now she balances on one leg (she’s a very clever elephant).

What is the pressure new pressure?

Pressure =

= N/cm2
Well done. You've got them all right.

You can see the results in the table. What happens to the pressure as the area gets bigger?

Area
N
Pressure
N/cm2
1200 10
600 20
300 40

The pressure gets smaller as the area gets bigger. This is because area is on the bottom of the fraction.

In fact, when the area doubles, the pressure halves. And when the area quadruples, the pressure is quartered. The pressure goes down in the same proportion that the area goes up. We say that the pressure is inversely proportional to the area.