NCERT Solutions for Class 8th: Ch 11 Force and Pressure Science

NCERT Solutions for Class 8th: Ch 11 Force and Pressure Science

Page No: 142

Excercise

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

Answer

Two examples of push force are as follows:
→ A heavy box at rest is pushed to move it from one room to another. This changes the state of motion of the box.
→ 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:
→ Rope is pulled to draw water from a well. This changes the state of motion of the water bucket.
→ A drawer is pulled to open it. This changes the state of motion of the drawer.

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

Answer

Two examples of forces that cause a change in the shape of an object are as follows:
→ Making of bread from dough
→ Deformation of clay by pressing it between the hands.

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.

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 push or pull it.
(d) The north pole of a magnet repels the north pole of another magnet.

Page No: 143

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.

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.

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.

(c) A load suspended from a spring while its other end is on a hook fixed to a wall.

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

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

When a blacksmith hammers a hot piece of iron, he uses his muscular force. This muscular force changes the shape of the iron so that it can be given a desired shape.

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 is responsible for the attraction between the balloon and the wall.

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.

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

The two forces acting on the rocket are the force of gravity, which pulls the rocket towards the ground, and the force of friction due to earth's atmosphere, which opposes its motion.

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


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