ICSE Class 6 Physics Chapter 4 Pressure

4 Pressure

Pressure is the force acting per unit area. Simple calculations based on the formulae:

Pressure = Force/Area i.e., P = F/A. A few examples from daily life.

Pressure

Let us try to know how things move and how the moving things come to rest. A door cannot open on its own unless some pull or push is applied to it. Similarly, to move a ball we need to kick it, throw it or hit it. On the other hand, to stop a moving ball coming towards you, you either catch it or stop it by your foot or hand or body. To sum up we say, to make an object move or to bring a moving object to rest, we have to apply force. Thus, force is the push or pull applied to move or stop a moving object.

The effect or magnitude of force applied to an object depends upon the area of the surface on which it acts. For the same force, this effect is less if it acts on a larger area while the effect is more if force acts on a smaller area.

a surface is called pressure and it is written as,

Pressure (P) = Force (F)/Area (A) or P = F/A

Pressure Depends On:

(a) The magnitude of the force applied i.e., greater the magnitude of force more is the pressure.

(b) The area over which the force is applied i.e., bigger the area on which the force is applied, lesser will be the pressure (Fig. 4.2).

Thus, for a fixed area of contact, the pressure exerted increases with increase in force.

And for a given force, greater the area of contact, lesser is the pressure exerted by it.

Activity 1

1. Take two rectangular blocks of iron having the same weight. Place them in mud as shown in Fig. 4.2(a). Which block X or Y will sink into the mud deeper?

2. Now place both blocks sidewise and keep an additional block of the same size and weight over block Y as shown in Fig. 4.2(b). Why does Y sink further into the mud?

Unit of Pressure

In S.I. units, pressure is measured in newton per square metre or N/m². One newton per square metre is called one Pascal and is expressed as Pa. This unit has been derived from the unit of force i.e., Newton (N) and unit of area i.e., square metre (m²).

Unit of Pressure = 1 Newton/1 m² = 1 N/m² or 1 N m-² = 1 Pascal or 1 Pa.

The bigger unit of pressure is called kilo Pascal (kPa).

1 kPa = 1000 Pascals

Blaise Pascal was a physicist as well as a mathematician. He did a lot in the field of atmospheric pressure, hence the unit of pressure was named after him.

Knowledge bank

Commonly the unit 1 atm is taken as the unit of pressure

1 atm = 76 cm of mercury column = 760 mm of mercury column

Pressure Depends on Force and Area

Let us conduct the following activity to understand the concept of pressure more precisely.

Activity 2

Push a sharp pin into a piece of wood as shown in Fig. 4.3. Also, try to push a nail with your thumb

into the wood. Even when you apply the maximum force by your thumb you cannot push the nail into the wood. Why?

Obviously, the difference has something to do with the area of the tip of the pin and the nail. Though the force applied is the same, the tip of the pin has a smaller area, hence the force acts on a smaller area and puts a larger pressure. However, if you press the nail with your thumb, the force is spread over a larger area and therefore, the pressure is less and hence it does not push the nail into the wood. Now take another nail and push it down with the help of a hammer. The nail will go deeper as a larger force is applied on the nail.

The above activity indicates that pressure depends on two factors - area and force. Pressure increases if: (a) the area decreases or (b) the force increases.

Intext Questions

1. Write the formula to calculate pressure.

2. The effect of force depends on which factors?

3. Give an example of exerting pressure in your daily life.

4. Where is the pressure due to water higher in a dam - near its base or near its top?

Examples Related to Area and Pressure

To understand the fact that the decrease in area increases the pressure and conversely, the increase in area decreases the pressure, let us consider the following examples:

1. A nail or a board pin has one end pointed and sharp while the other end is blunt and flat. On applying force, the pointed end will exert greater pressure as the area of contact is small and hence it will pierce into the given surface.

2. The cutting instruments like a blade, knife, axe, etc., have very sharp edges. The sharp edges have very small area of contact, so the pressure is more. Hence, they can easily penetrate through the given surface.

3. Heavy trucks have six to eight tyres instead of the conventional four. More number of tyres are used to increase the area of contact and thereby reduce the pressure on the ground.

4. A camel can move more conveniently on sand as compared to a horse, due to the fact that the camel has broader feet than a horse. The broader feet of the camel provide lesser pressure on the sand and it becomes easier for the camel to walk. In the case of a horse, the area of the feet is less, due to which the pressure increases and hence the feet show a tendency to sink inside the sand, making it difficult to walk.

5. Skiers use long flat skis to slide over the snow. The larger the area of contact, the lesser is the pressure on the snow. This helps the skier to slide comfortably without sinking in the snow.

6. Army tanks are usually very heavy and they exert large amount of pressure on the ground, if move on wheels. Hence, they are made to move over broad steel tracks called caterpillar wheels of tanks rather than an ordinary wheels. The steel tracks are used to increase the surface area which reduces the pressure on the ground and hence avoid sinking of the wheels in the ground.

7. Foundation of buildings are kept wide so that the weight of the building may act on larger area. As a result it will exert less

Teacher's Note

Pressure in daily life is everywhere - from the pins holding up a poster to the wide tires on trucks, understanding how force distributes across different areas helps us design safer and more efficient structures and tools.

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