NCERT Solutions for Class 8 Science Chapter 11 Force and Pressure
Question:- 1. Give two examples each of situations in which you push or pull to change the state of motion of objects.
Answer:- (i) Push: A football is pushed by a player’s foot.
We push a wooden box around.
(ii) Pull: We pull the rope to get water from a well.
A cart is pulled by a horse.
Question:- 2. Give two examples of situations in which applied force causes a change in the shape of an object.
Answer:- Two examples of situations in which applied force causes a change in the shape of an object are:-
Stretching of a rubber band.
Moulding of clay.
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.
(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 push or pull it.
(d) The north pole of a magnet repels the north pole of another magnet.
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 thefollowing 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.
(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:- Agents – fingers
Object – lemon
Effect of force – changes the shape of lemon.
(b) Taking out paste from a toothpaste tube.
Answer:- Agents – fingers
Object – toothpaste tube
Effect of the force – can be observed as the paste coming out of the tube change in shape.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
Answer:- Agent – load suspended
Object – spring
Effect of the force – can be seen in the form of elongation of spring on suspension of load (change in shape).
(d) An athlete making a high jump to clear the bar at a certain height.
Answer:- Agent – muscles of athlete
Object – athlete himself
Effect of the force – changes the state of motion of the 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:- While hammering a hot piece of iron, a blacksmith uses his muscular force. The muscular force changes the shape of the iron to his desired shape.
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:- We use muscular force to lift the bucket of water above the ground. This muscle force opposes force of gravity on the bucket, which pulls it towards the earth. The magnitude of the two forces is equal, but their directions are opposite. As a result, the bucket’s net force is zero.Hence it do not bring a change in its 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.
Answer:- When a rocket leaves the launch pad, the following forces are act:-
(i) Earth’s gravitational pull (downward)
(ii) Air frictional force (in opposite direction)
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.
Answer:- (d) atmospheric pressure.