NCERT Solutions Class 8 Science Chapter 11 Force and Pressure

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

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

Answer:

Examples of Push:

(i) A suitcase at rest is pushed  to move  from one place to another. This changes  the state of motion of the suitcase.

(ii) A football is pushed  by a player using  his foot. This changes  the state of motion of the football.

Examples of Pull:

(i)  To open  a drawer, it is pulled  out. This changes the state of motion of the drawer.

(ii) Rope is pulled  to draw water from a well. This changes  the state of motion of the water bucket.

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

Answer:

(i) Squeezing of tin container.
(ii) Pressing clay by 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  either push orpull it.

(d)    The north pole of a magnet  repels the north pole of another magnet.

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 of muscular, contact, non-contact, gravity, friction, shape, attraction, etc.

(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.

(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.

Answer:

(a)    Muscular force is used in squeezing a piece of lemon. This muscular force acts on the lemon. Thus, the shape  of the lemon gets changed.

(b)    To take out paste from a toothpaste tube, muscular force is used and it acts on the toothpaste tube. Thus, the shape  of the tube gets changed.

(c)    The suspended load exerts a force on the spring and pushes  the spring in downward direction. Thus, the spring gets stretched and its shape  gets changed.

(d)    In a high  jump, an athlete pushes  the ground with his feet and exerts a muscular force on the ground. As a result, he can jump over the bar. Thus, his state of motion gets changed.

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: A blacksmith uses muscular force when  he hammers on a hot piece of iron. Due to this muscular force, the shape  of the iron gets changed and iron piece 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: An inflated balloon gets charged when  it is rubbed with a piece of synthetic cloth. A charged body  attracts an uncharged body. When  this balloon is pressed against a wall, it sticks to the wall. The force acting between the charged balloon and the wall  is the electrostatic force due to which  it was stuck.

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: When  we hold  a plastic bucket filled  with water above  the ground, there are two forces act on it. First is the gravitational force which  acts downwards and second  is the muscular force exerted by us which  acts in the upward direction. The two forces are equal  in magnitude but  opposite in direction. Hence, the net force on the bucket is zero. Therefore, there is no change  in its state of motion.

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:

(i) The force of gravity which  tries to pull  the rocket downwards.

(ii) The force of friction due to earth’s atmosphere, which  opposes  the motion of the rocket.

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 dropp 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: The correct option is (d).

Explanation: When  the air filled  in the nozzle  is removed, the atmospheric pressure, which  is acting on the water, forces the water to fill  the nozzle  of the dropper.