## Topic outline

• ### Force and Pressure

##### Q62. If the area of my head were 10 cm × 10 cm, how much weight of air would I be carrying on my head? Why don't we feel the weight of such a huge amount of air on us?

Ans. 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.

##### Q63. If a vacuum is created between two Magdeburg hemispheres joined together, they cannot be separated easily. Give reason.OrWhy do Magdeburg hemispheres stick together?

Ans. This is due to the fact that since there is no air inside, the unopposed atmospheric pressure acting over the whole surface of the hemispheres from outside presses them very, very hard and does not allow them to be separated.

##### Q64. How does a rubber sucker work?

Ans. When we 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. 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.

##### Q66. Give two examples each of situations in which you push or pull to change the state of motion of objects.

Ans. Objects are pushed or pull in order to change the state of motion.

Examples of push are:

i.        We push the door to open it.

ii.        A football player kicks the ball.

Examples of pull are:

i.        We pull the door to close it.

ii.        Opening a drawer.

##### Q67. When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to(a) pressure of water(b) gravity of the earth(c) shape of rubber bulb(d) atmospheric pressure.

Ans. The rise of water in the dropper is due to atmospheric pressure.

##### Q69. An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall?

Ans. When inflated balloon is rubbed with a piece of synthetic cloth, it gets electrically charged. A charged body exerts an electrostatic force on uncharged body. Thus, when the charged balloon is pressed against a wall, it sticks to the wall because of the electrostatic force acting between the charged balloon and the wall.

##### Q70. Explain why, when a person stands on a cushion, the depression is much more than when he lies down on it.

Ans. When a man stands on a cushion then only his two feet are in contact with the cushion. Due to this, the weight of man falls on a small area of cushion producing a large pressure. This large pressure causes a big depression in the cushion. On the other hand, when the same man is lying on the cushion, then his whole body is in contact with the cushion. In this case, the weight of man falls on a much larger area of the cushion producing much smaller pressure. And this smaller pressure produces a very little depression in the cushion.