Form 4 Chapter 3
Force and Pressure

3.1 Understand Pressure

You should be able to

1. Conceptulise pressure as force over area.

2. solve quantitative problems involving pressure.

3. describe applications of pressure.

Common questions:

1. What is the relationship between the pressure and surface area?

The smaller the surface area, the bigger the pressure.

2. What is the relationship between the force and the pressure?

The bigger the force, the bigger the pressure.

3. What is the SI unit for pressure?

Nm^-2 or Pascal (Pa).

4. The applications of pressure in daily life. Examples :

• The blade of a knife has a very small area that exerts such a high pressure that the meat can be cut easily.

• The sharp tip of a thumbtack has a very small area in contact with the board. Hence the tip can pierce through the board due to the high pressure it exerts.

• You will not sink into the snow if you are wearing skis. The pressure exerted is low due to the large surface area of the skis.

• The large tracks of a bulldozer help to reduce pressure so that the heavy bulldozer will not sink into the soft ground.

Simple calculations:

1. The diagram below shows the dimension and the weight of a wooden block resting on the floor.

Calculate

(i) the maximum pressure exerted, and

(ii) the minimum pressure exerted.

2. A student is pressing a thumbtack into a notice board with a force of 15 N. The area of the head and the tip of the thumbtack is 0.0001 m² and 1×10^-6 m² respectively.

(i) Calculate the pressure exerted on the student’s thumb.

(ii) Calculate the pressure exerted by the thumbtack onto the notice board.

(iii) What causes the difference in pressures calculated in 2(i) and 2(ii)?

3. A four legged table of 200 N has a total cross sectional area of 40 cm². What is the average pressure exerted by each leg of the table to the floor?