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NCERT Solutions for Class 8 Science chapter 11 – Force and Pressure


Back Exercise

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

Answers
Two examples of push force are as follows:
(i) A heavy box at rest is pushed to move it from one room to another. This changes the state of motion of the box.
(ii) A player pushes a football using his foot. This changes the state of motion of the ball.

Two examples of pull force are as follows:
(i) Rope is pulled to draw water from a well. This changes the state of motion of the water bucket.
(ii) A drawer is pulled to open it. This changes the state of motion of the drawer.

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

Answers
Two examples of forces that cause a change in the shape of an object are as follows: (i) Squeezing of a plastic bottle changes the shape of the bottle. (ii) Deformation of clay by pressing it between the hands.

Question 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.

Answers
a) pull
(b) attracts
(c) push or pull
(d) repels

Question 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.

Answers
(a) shape
(b) muscular
(c) contact
(d) gravity, friction

Question 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.

Answers
(a) Agent exerting the force – Fingers
Object on which force acts – Lemon
Effect of force – Change in shape of lemon

(b) Agent exerting the force – Fingers
Object on which force acts – toothpaste tube
Effect of force – Change in shape of toothpaste tube.

(c) Agent exerting the force – Load
Object on which force acts – Spring
Effect of force – Change in shape of spring.

(d) Agent exerting the force – Muscles of Athlete
Object on which force acts – Athlete
Effect of force – Change of state of motion of athlete

Question 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?

Answers
The force due to hammering changes the shape of the piece of iron.

Question 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?

Answers
The force which is responsible for the attraction between the ball and the wall is electostatic force. When we rub the balloon by a synthetic cloth, it get charged. When it is taken near the wall, it get attracted towards the uncharged wall (charged body attracts uncharged body) because of the electostatic force.

Question 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.

Answers
For holding the bucket of water above the ground, we use muscular force. 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 the state of motion.

Question 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.

Answers
The force of gravity is the one which acts on the rocket to pull it towards the ground and the other one is the force of friction due to earth’s atmosphere, which opposes its motion.

Question 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.

Answers
(d) atmospheric pressure