Topic outline

    • Force and Pressure

      Q51. Give two examples of situations in which applied force causes a change in the shape of an object.

      Ans. Two examples in which applied force causes a change in the shape of an object are:

                                i.        The shape of dough changes on pressing with rolling pin to make chapatis.

                               ii.        The shape of a toothpaste tube changes when we squeeze it.


      Q52. What are the various effects of force?

      Ans. Effects of force are:

                                i.        It may make an object move from rest.

                               ii.        It may change the speed of an object if it is moving.

                              iii.        It may change the direction of motion of an object.

                              iv.        It may bring about a change in the shape of an object.

      Q53. Why a fountain of water is created at the leaking joints or holes of pipes of the main water supply line?

      Ans. Many times we see a fountain of water rushing out of the leaking joints (or holes) in the pipes of main water supply line. It is due to the very high pressure exerted by water on the sides (or walls) of the pipes that such a fountain of water is formed.


      Q54. Why does the liquid rise in a syringe when its piston is pulled up?
      How does a liquid medicine enter a syringe?

      Ans. When the nozzle of a syringe is dipped in a liquid and its piston is withdrawn, the pressure inside the syringe is lowered. The greater atmospheric pressure acting on the surface of the liquid pushes the liquid up into the syringe.


      Q55. Why nose bleeding occurs at high altitudes?
      Why do mountaineers suffer from nose bleeding at high altitudes?

      Ans. At high altitudes, the atmospheric pressure becomes much less than our blood pressure. Since, our blood is at higher pressure than outside pressure, therefore, some of the blood vessels in our body burst and nose bleeding takes place at high altitudes.


      Q56. Does application of a force would always result in a change in the state of motion of the object?

      Ans. No. Many a time application of force does not result in a change in the state of motion. For example, a heavy box may not move at all even if you apply the maximum force that you can exert. Again, no effect of force is observed when you try to push a wall.

      Q57. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

      Ans. In this case, muscular force is acting in upward direction and the force of gravity is acting in downward direction. The two forces are equal in magnitude but opposite in direction. Therefore, the net force on the bucket is zero. Hence, there is no change in its state of motion.


      Q58. Why atmospheric pressure decreases as we go higher up above the earth's surface?

      Ans. The pressure at any level in the atmosphere may be interpreted as the total weight of the air above a unit area at any elevation. Since most of the atmosphere's molecules are held close to the earth's surface by the force of gravity, air pressure decreases as we go higher up above the earth's surface.


      Q59. What happens when a charged plastic straw is brought near another uncharged plastic straw?

      Ans. A straw is said to have acquired electrostatic charge after it has been rubbed with a sheet of paper. Such a straw is an example of a charged body. A straw rubbed with paper attracts another straw but repels it if it has also been rubbed with a sheet of paper.


      Q60. Why does a plastic comb rubbed with dry hair attract tiny pieces of paper?

      Ans. When a plastic comb is rubbed in dry hair, the comb gets electric charges by friction. And when this electrically charged comb is brought near tiny pieces of paper, it exerts an electrostatic force of attraction on them. Due to this electrostatic force, the tiny pieces of paper move towards the comb and stick to it.

      Q61. What is meant by saying that force is due to interaction?

      Ans. Example: Suppose a man is standing behind a stationary car. The car will not move just due to his presence. Suppose the man now begins to push the car, that is, he applies a force on it. The car may begin to move in the direction of the applied force.

      From the above example, we can infer that at least two objects must interact for a force to come into play.

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