Class 8 Science

Force and Pressure

NCERT Solution

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

Answer: Push:

To close a drawer
To move a wooden box on floor

Pull:

To open a drawer
A horse pulling a cart

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

Answer: Making of bread from dough
Stretching of a rubber band

Question 3: Fill in the blanks in the following statements:

To draw water from a well we have to ——— at the rope.
A charged body ———— an uncharged body towards it.
To move a loaded trolley we have to ———— it.
The north pole of a magnet ———— the north pole of another magnet.

Answer: (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)

To stretch the bow, the archer applies a force that causes a change in its ________.
The force applied by the archer to stretch the bow is an example of — —— force.
The type of force responsible for a change in the state of motion of the arrow is an example of a ———— force.
While the arrow moves towards its target, the forces acting on it are due to ———— and that due to ———— of air.

Answer: (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.

Answer: Agent exerting the force – Fingers
Object on which force acts – Lemon
Effect of force – Change in shape of lemon

(b) Taking out paste from a toothpaste tube.

Answer: Agent exerting the force – Fingers
Object on which force acts – toothpaste tube
Effect of force – Change in shape of toothpaste tube.

Answer: Agent exerting the force – Load
Object on which force acts – Spring
Effect of force – Change in shape of spring.

(d) An athlete making a high jump to clear the bar at a certain height.

Answer: 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?

Answer: 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?

Answer: Electrostatic 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.

Answer: In this case, muscular force is acting in upward direction and the force of gravity is acting in downward direction. Both forces are acting in opposite directions to each other and hence nullify the effect of each other. Due to this, there is no change in the state of motion of the bucket.

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.

Answer: Gravitational force and atmospheric pressure.

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

pressure of water
gravity of the earth
shape of rubber bulb
atmospheric pressure

Answer: (d) Atmospheric pressure

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