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.
      Why 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.


           Q65. Identify the actions involved in the following situations as push or pull, or both:

             a.   Opening a drawer. Pull
             b.   A cricket ball hit by a batsman. Push
             c.   Drawing a bucket of water from a well. Pull
             d.   Moving a book placed on a table. Push; Pull
             e.   A football player taking a penalty corner. Push
             f.    Moving a wheel barrow. Push; Pull


      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.


      Q68. Name the type of forces involved in the following:
      a. A horse pulling a cart. Muscular force
      b. Moving a loaded cart. Muscular force
      c. A sticker attached to steel almirah without glue. Magnetic force
      d. A coin falling to the ground on slipping from hand. Gravitational force
      e. A plastic comb rubbed in dry hair picking up tiny pieces of paper. Electrostatic force
      f. A moving boat coming to rest when rowing is stopped. Frictional force
      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.


      Q71. An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information fill up the gaps in the following statements using the following terms: muscular, contact, non-contact, gravity, friction, shape, attraction
      (a) To stretch the bow, the archer applies a force that causes a change in its shape.
      (b) The force applied by the archer to stretch the bow is an example of muscular force.
      (c) The type of force responsible for a change in the state of motion of the arrow is an example of a contact force.
      (d) While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction of air.

      Q72. In the following situations identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.
      (a) Squeezing a piece of lemon between the fingers to extract its juice.
      (b) Taking out paste from a toothpaste tube.
      (c) A load suspended from a spring while its other end is on a hook fixed to a wall.
      (d) An athlete making a high jump to clear the bar at a certain height.


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