Force And Pressure - Class 8 : NCERT Exercise Questions



 

Q.1 Give two examples each of situations in which you push or pull to change the state of motion of objects.
Sol. Two examples in which pushing change the state of motion are:
(i) Hitting ball at rest.
(ii) A heavy box pushed to move it from one place to another.
Two examples in which pulling change the state of motion are:
(i) Drawing water from well.
(ii) Opening a cupboard.

Q.2 Give two examples of situations in which applied force causes a change in the shape of an object.
Sol. Two examples in which applying force causes change in the shape of an object are:
(i) Squeezing toothpaste.
(ii) Stretching a rubber belt.

Q.3 Fill in the blanks in the following statements:
(a) To draw water from a well we have to __________ at the rope.
(b) A charged body __________ an uncharged body towards it.
(c) To move a loaded trolley we have to __________ it.
(d) The north pole of a magnet __________ the north pole of another magnet.
Sol. (a) To draw water from a well we have to pull at the rope.
(b) A charged body attracts an uncharged body towards it.
(c) To move a loaded trolley we have to either push or pull it.
(d) The north pole of a magnet repels the north pole of another magnet.

Q.4 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 _________.
(b) The force applied by the archer to stretch the bow is an example of _________ force.
(c) The type of force responsible for a change in the state of motion of the arrow is an example of a _________ force.
(d) While the arrow moves towards its target, the forces acting on it are due to _________ and that due to _________ of air.
Sol. (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.

Q.5 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.
Sol. (a) In squeezing a piece of lemon between the fingers to extract its juice, we make use of muscular force. The force is applied on the lemon and, as a result, the shape of lemon gets changed.
(b) In taking out paste from a toothpaste tube, muscular force is used. The force is exerted on the toothpaste tube and, as a result, shape of tube gets changed.
(c) In this case, the suspended load exerts a force on the spring and pushes the spring downwards. As a result, the spring gets stretched.
(d) In this case, the feet of an athlete exert a muscular force on the ground and push the ground. This allow athlete to jump high and his state of motion gets changed.

Q.6 A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?
Sol. When a blacksmith hammers a hot piece of iron to make a tool, he exerts muscular force on it. This muscular force changes the shape of iron to give it the desired shape.

Q.7 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?
Sol. An electrostatic force is responsible for the attraction between balloon and wall. When an inflated balloon is rubbed with a piece of synthetic cloth, it becomes charged. A charged body attracts an uncharged body. Hence, on pressing balloon against a wall, it sticks to the wall.   

Q.8 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.
Sol. The two forces acting on a plastic bucket containing water held above ground level in hand are:
(i) Gravitational Force: It is force acting in the downward direction.
(ii) Muscular Force: It is the force applied by our hands to lift bucket in the upward direction.
These two forces are equal in magnitude and opposite in direction, therefore, the net force is zero. Hence, there is no change in bucket’s state of motion.

Q.9 A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.
Sol. The two forces on the rocket immediately after leaving the launching pad are:
(i) Gravitational Force: It is force acting on rocket pulling it in the downward direction.
(ii) Frictional force: It is the force acting due to presence of earth’s atmosphere, which opposes its motion in upward direction.

Q.10 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
Sol. (d) atmospheric pressure.             



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