WHAT IS PRESSURE?
You have learned in Class VII that strong winds during a storm or a cyclone can blow away even the roof-tops. You also learned that winds and cyclones are caused by differences in air pressure. Is there any relation between pressure and force? Let us find out.
Try to push a nail into a wooden plank by its head. Did you succeed? Try now to push the nail by the pointed end (Fig. 11.12). Could you do it this time?
Figure 11.12: Pushing nail into the wooden plank
Try cutting vegetables with a blunt knife and then with a sharp knife. Which is easier?
Do you get the feeling that the area over which the force is applied (for example, the pointed end of the nail) plays a role in making these tasks easier?
The force acting on a unit area of a surface is called pressure.
pressure = force / area on which it acts
At this stage, we consider only those forces which act perpendicular to the surface on which the pressure is to be computed.
You now understand why porters place a round piece of cloth on their heads when they have to carry heavy loads (Fig. 11.13). By doing this they increase the area of contact of the load with their head. So, the pressure on their head is reduced and they find it easier to carry the load.
Figure 11.13: A porter carrying a heavy load
Note that the area is in the denominator in the above expression. So, the smaller the area, larger the pressure on a surface for the same force. The area of the pointed end of the nail is much smaller than that of its head. The same force, therefore, produces a pressure sufficient to push the pointed end of the nail into the wooden plank.
Can you explain now why shoulder bags are provided with broad straps and not thin strap? And, why the tools meant for cutting and piercing always have sharp edges?
Do liquids and gases also exert pressure? Does it also depend on the area on which the force acts? Let us find out.
Source: This topic is taken from NCERT TEXTBOOK