Pressure

Objective

This topic gives an overview of;

• Pressure
• Pressure exerted by Liquids and Gases
• Atmospheric Pressure

Pressure

Strong winds during a storm or a cyclone can blow away even the roof-tops. Winds and cyclones are caused by the differences in air pressure. Try to push a nail into a wooden plank by its head. Try now to push the nail by the pointed end. Try cutting vegetables with a blunt and a sharp knife.  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 is making these tasks easier? The force acting on a unit area of a surface is called pressure

At this stage we consider only those forces which act perpendicular to the surface on which the pressure is to be computed.

Pressure = Force / Area on which it acts

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.

Shoulder bags are provided with broad straps and not thin strap.The tools meant for cutting and piercing always have sharp edges because  the larger  the area,the pressure would be less.

This is why porters place on their heads a round piece of cloth, when they have to carry heavy loads . By 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.

Pressure exerted by Liquids and Gases

Take a transparent glass tube or a plastic pipe. The length of the pipe / tube should be about 15 cm and its diameter should be 5-7.5 cm. Also take a piece of thin sheet of a good quality rubber, say, a rubber balloon. Stretch the rubber sheet tightly over one end of the pipe. Hold the pipe at the middle, keeping it in a vertical position.  Ask one of your friends to pour some water in the pipe. Note also the height of the water column in the pipe. Pour some more water. Observe again the bulge in the rubber sheet and the height of the water column in the pipe. Repeat this process a few more times. You will see a relation between the amount of the bulge in the rubber sheet and the height of the water column in the pipe.

Take a plastic bottle. You can take a discarded water or soft drink bottle. Fix a cylindrical glass tube, a few cm long near its bottom. You can do so by slightly heating one end of the glass tube and then quickly inserting it near the bottom of the bottle. Make sure that the water does not leak from the joint. If there is any leakage, seal it with molten wax. Cover the mouth of the glass tube with a thin rubber sheet. Now fill the bottle upto half with water. Pour some more water in the bottle.There is a change in the bulge of the rubber sheet.

Note that the rubber sheet has been fixed on the side of the container and not at the bottom. The bulging of the rubber sheet in this case indicate that water exerts pressure on the sides of the container as well. Let us investigate further.

Take an empty plastic bottle or a cylindrical container. You can take a used tin of talcum powder or a plastic bottle. Drill four holes all around near the bottom of the bottle. Make sure that the holes are at the same height from the bottom. Now fill the bottle with water.

Fountains of water coming out of the leaking joints or holes in pipes supplying water. It is due to the pressure exerted by water on the walls of the pipes.

When you inflate a balloon, why do you have to close its mouth? What happens when you open the mouth of an inflated balloon? Suppose you have a balloon which has holes. Would you be able to inflate it? If not, why? Can we say that air exerts pressure in all directions? Do you recall what happens to the air in the bicycle tube when it has a puncture? Do these observations suggest that air exerts pressure on the inner walls of an inflated balloon or a tube? So, we find that gases, too, exert pressure on the walls of their container.

Atmospheric Pressure

You know that there is air all around us. This envelop of air is known as the atmosphere. The atmospheric air extends up to many kilometres above the surface of the earth. The pressure exerted by this air is known as atmospheric pressure. We know that pressure is force per unit area. If we imagine a unit area and a very long cylinder standing on it filled with air, then the weight of the air in this cylinder is the atmospheric pressure.

When you press the sucker, most of the air between its cup and the surface escapes out. The sucker sticks to the surface because the pressure of atmosphere acts on it. To pull the sucker off the surface, the applied force should be large enough to overcome the atmospheric pressure. This activity might give you an idea about the magnitude of atmospheric pressure. In fact, it would not be possible for any human being to pull the sucker off the surface if there were no air at all between the sucker and the surface.

If the area of my head were 10 cm × 10 cm, how much weight of air would I be carrying on my head ? The weight of air in a column of the height of the atmosphere and area 10 cm × 10 cm  is as large as 1000 kg. The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric pressure and cancels the pressure from outside.

Otto von Guericke, a German scientist of 17th century, invented a pump to extract air out of the vessel. With the help of this pump, he demonstrated dramatically the force of the air pressure. He joined two metallic hemispheres of 51 cm diameter each and pumped air out of them. Then he employed eight horses on each hemisphere to pull them apart . So great is the force of air pressure that the hemispheres could not be pulled apart.

Summary

• Force per unit area is called pressure.
• The thrust acting per unit surface area is pressure.
• The force acting on an object perpendicular to its surface is thrust
• Pressure is measured in newton/metre2, which is equal to 1 pascal (Pa).
• Liquids and gases exert pressure on the walls of their containers.
• The pressure exerted by air around us is known as atmospheric pressure.

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