Notes of chapter: Presure are presented below. Indepth notes along with worksheets and NCERT Solutions for Class 8.
The perpendicular force acting upon unit area of a surface is called pressure.
Pressure = Force / area on which it acts
If surface area is smaller pressure is larger.
If surface area is larger pressure is smaller.
(i) Head of nail is larger than pointed tail of the nail. Therefore, when we put force on the pointed tail of the nail, nail pushed into the wooden plank or wall.
But, head of the nail has larger surface area. Therefore, it is not easy to push head of the nail into the wooden plank.
(ii) Our shoulder bags have broad straps because large surface area exert less pressure of bag on the shoulder.
(2) Pressure exerted by liquids-
(i) Pressure exerted by the liquids at the bottom of the container depends on the height of the column in which water or liquids has kept
Take a transparent glass tube or plastic pipe and a rubber balloon.
Now pour water in to the tube. The rubber balloon or sheet will bulge out. Now pour more water into the tube. The bulge of rubber balloon will be extended.
Therefore, it is clear that pressure exerted by the water or liquids depends upon the height of water in which it is kept.
(ii) A liquid exerted pressure on the walls of the container-
Take a transparent water bottle, a cylindrical glass tube and a balloon or rubber sheet. Now, heat one end of the cylindrical glass tube and fix it just above the bottom of the bottle. Fix balloon on the other end of the cylindrical glass tube. Now pour some water in to the bottle, the balloon will bulge out. As we pour more water in to the bottle, balloon bulge will increase. Therefore, it shows that liquid exert pressure on the walls of the container.
(iii) Liquids exert equal pressure at the same depth-
Take a transparent water bottle. Do four holes near the bottom of the bottle. All holes should be at the same height. Now fill the bottle with water. The four water streams will be out from the bottle. These streams fall at the same distance from the bottle. Therefore, liquids exert equal pressure at the same depth.
(3) Pressure exerted by the gases-
(i) Gases also exerted pressure on the walls of their container.
We have to tie the mouth of the inflated balloon. Otherwise, air or gas will be released from the balloon.
(ii) We can not inflate a balloon with holes because all air will be immediately released by the balloon through holes.
(4) The envelop of the air around us is known as atmosphere. The pressure exerted by the air of atmosphere on earth is known as atmospheric pressure.
(i)Atmospheric pressure is the pressure exerted by the weight of the air above a unit area.
Eg:- If a very long cylinder standing on the unit area is filled with air, then the weight of the air in this cylinder is the atmospheric pressure.
(ii) We have to put force more than atmospheric pressure exerted on it to pull it.
We press sucker and air between the cup and surface come out and sucker sticks. But when we want to remove it from the surface we have to put force more than the atmospheric pressure. If there is no air between the sucker and the surface we can not pull the sucker at all.
(iii) The pressure on our head is very high but we did not crushed by this pressure. Because pressure inside our bodies is also equal to the atmospheric pressure and cancels the pressure from outside.