NCERT solutions for class 8 Science Chapter 11 Force And Pressure

Solution for Exercise

Q.1 Give two examples each of the situations in which you push or pull to change the state of motion of objects.


Open in new tab link

Answer :

1. push situation

(a) We close drawer by pushing.
(b) We move a wooden box by pushing.

2. Pull situation

(a) We draw water from a well by pulling the rope.
(b) A horse pulls a cart.

Q.2 Give two examples of situations in which applied force causes a change in the shape of an object.


Open in new tab link

Answer :

The forces which change the shape of an object are as follows:
i) By pressing the clay between the hands, it deforms.
ii) When we apply force on a rubber band to stretch it

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.


Open in new tab link

Answer :

(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 __pull or push__ 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.


Open in new tab link

Answer :

(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.


Open in new tab link

Answer :

(a) In squeezing a piece of lemon, we make use of muscular force to extract its juice. This muscular force acts on the lemon. As a result, the shape of the lemon gets changed.
(b) We use our muscular force to take out paste from a toothpaste tube. The muscular force acts on the toothpaste tube. As a result, the shape of the tube gets changed.
(c) Here, the suspended load exerts a force on the spring and pushes the spring downwards. As a result, the spring gets stretched. Hence, its shape gets changed.
(d) Agent is muscles of athlete, object is athlete himself and effect of the force changes the state of motion of the athlete.

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?


Open in new tab link

Answer :

The force due to hammering causes the change in the shape of the iron and iron can be moulded in the shape of the required tool.

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?


Open in new tab link

Answer :

Electrostatic force

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.


Open in new tab link

Answer :

We make use of muscular force to hold a bucket of water above the ground. This muscular force acts against the force of gravity that pulls the bucket towards the ground. 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.

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.


Open in new tab link

Answer :

The forces that act when a rocket leaves launching pad are Gravitational force of the earth (downward) and Frictional force of air (in opposite 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


Open in new tab link

Answer :

Atmospheric pressure



Get it on Google Play