3.1 PRESSURE

define pressure and state that P = F/A

describe applications of pressure

solve problems involving pressure

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LEARNING OUTCOMES:

Define Pressure and state its formula

• Pressure = ForceArea

The SI unit : Nm-2 = Pascal = Pa

A

FP

State relationship between pressure and area.

The pressure of a given force increases as the surface area decreases.

Solve problems

The lower the

surface area, the

higher the pressure

Application of High PressureIncreasing the pressure by reducing the area

• A sharp knife has a very small surface area on its cutting edge so that high pressure can be exerted to cut the meat.

Application of High PressureIncreasing the pressure by reducing the area

The studs on a football boot have only a small area of contact with the ground. The pressure under the studs is high enough for them to sink into the ground, which gives extra grip.

• Nails, needles and pins have very sharp ends with very small surface areas. When a force is applied to the head of a nail, the pressure will drive its sharp end into a piece of wood easily.

Application of High PressureIncreasing the pressure by reducing the area

Application involving Low PressureReducing the pressure by increasing the area

• Skis have a large area to reduce the pressure on the snow so that they do not sink in too far.

• A tractor moving on soft ground has wide tires to reduce the pressure on the ground so that they will not sink into the ground.

Application involving Low PressureReducing the pressure by increasing the area

Wide tires

• A wide shoulder pad of a heavy bag will reduce the pressure exerted on the shoulder of the person carrying the bag.

Application involving Low PressureReducing the pressure by increasing the area

Wide shoulder pad

As a shop assistant, you are asked to place some nails bought by a customer in a plastic bag. However, the plastic bag is very thin and the nails will likely poke through it. Suggest way to place the nails in the plastic bag so that the plastic bag will not be poked or torn by the nails. Explain your answer.

Wrap the nails in a newspaper.Increase the surface area so reduce pressure.

Figure (a) shows a boy sitting on a horizontal wooden bar. When he stands on the same horizontal bar as in Figure (b) it breaks.

(a)State one physical quantity that

(i) remains constant in both figure (a) and figure (b).

(ii) varies from figure (a) to figure (b).

(b) Explain why the bar does not break in figure (a) but breaks in figure (b).

Weight / forceArea, A / pressue P

In figure (b), the surface area is smaller, so high pressure.