Understanding Pressure

Pressure involves the concept of force and surface area.

1. Pressure on an area, A is the normal force, F, which is being applied perpendicularly to the area.

2. Pressure on an area, A is expressed as the normal force, F per unit area, A.

3. P = (F/A)

4. This SI unit for pressure is the pascal, Pa, where 1 Pa = 1 N/m2 (metre square).

5. Pressure is increased:

• if the force, F applied to a given area, A is increased
• if a given force, F is applied to a smaller area, A

6. If a balloon is pressed against by a finger, the balloon will only change its shape a bit.

If the balloon is pushed against by a needle with the same force, the balloon will burst.

This is because a finger has a larger surface area (A) than a needle. Hence, the needle exerts much pressure than the finger and perforates through the surface of the balloon, making a hole and freeing the air inside the balloon (or pops the balloon instead!)