FORCES AND PRESSURE

PRESSURE

• A force, F, applied normally to the surface with an area, A

P = F

A

The unit for pressure is Nm-2 or Pascal (Pa)

The pressure acted by a force on a surface depends on the area of the surface.

The pressure on the surface depends on the direction of the force ( only the component of the force that is perpendicular to the surface to determine the pressure)

APPLICATION OF PRESSUREVery heavy vehicles are designed with special wheels or tracks so that the large force is applied over a large of ground. This helps to reduce the pressure that is exerted

A knife is designed to have a thin cutting edge. The area of the edge is small so that the force can exert a very high pressure.

The base of high-heeled shoe is small; pressure greater; the base of flat shoe is large, pressure is small.

The sole of a football boot has sharp studs which come into contact with the ground at a high pressure. This gives the player a better grip of the ground

A nail can easily penetrate a wall because the sharp end of the nail exerts very high pressure.

SOLVING PROBLEMS

An elephant weighing 30 000 N stands on one foot. The area of the foot in contact with the ground is 1500 cm2. Calculate the pressure it exerts on the floor.Solution:

F= 30 000 N, A= 1500 cm2 = 1500 x 10-4m2.P = F = 30 000 = 200 000 Nm-2

A 1500 x 10-4

Hence, 200 kPa of pressure was exerted on the floor.

3 m3 m

10 m

Weight = 30N

10 m3 m

3 m

Weight = 30N Figure shows a box on a table in two different orientations. Calculate the pressure exerted on the table in each case

Case a

Case b

• case a• A1 = 3m x 3m• = 9m2

• P1= F/A1

• = 30N / 6m2

• = 5 Pa

• Case b• A2 = 10m x 3m• = 30 m2

• P2 = F/ A2

• = 30N / 30m2

• = 1 Pa

Then, what is your conclusion?

SUMMARIZE

PRESSUREP = F A

Effect of surface areaon pressure

Uses of pressurein everyday life

HOMEWORK1. A student pushed a 20 cent coin into a piece of plasticine in two ways as shown in diagram below

( a )

( b )

coins

plasticine

(i) If equal forces are applied in both the cases, compare the depth of the hollow formed by the coins.

(ii) Explain your observation in (i)

2. Two knives; a sharp knife and a blunt knife.

(a)Sharp knife

(b)Blunt knife

A force of 100N is applied on each knife to cut a cucumber.

(i) If the surface area of the blunt knife is 0.01cm2, calculate the pressure exerted on the cucumber.

(ii) If the surface area of the sharp knife is 0.01cm2, calculate the pressure exerted on the cucumber

3. On the table is a full box of apples which has a weight of 200N. The area of the box is 0.5 m2. What is the pressure between the box and the table top?

4. The weight of an elephant is 60 000 N. It has four feet. Each foot has an area of 500 cm2. What pressure does the elephant exert on the ground?

5. Siti weights 500N. She wears a pair of high-heeled shoes. The area of the heel of each high-heeled shoe is 5 cm2. What pressure do the heels exert on the ground?