NCERT Solutions for Class 8 Science Chapter 11 : Force and Pressure

Q:

A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

A:

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.

Q:

Fill in the blanks in the following statements.

(a) To draw water from a well we have to ___ on 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:

(a) To draw water from a well we have to pull on the rope.
(b) A charged body attracts an uncharged body towards it.
(c) To move a loaded trolley we have to either push or pull it.
(d) The north pole of a magnet repels the north pole of another magnet.

Q:

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.

A:

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.

Q:

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.

A:

  1. The force of gravity which tries to pull the rocket downwards.
  2. The force of friction due to earth’s atmosphere, which opposes the motion of the rocket.

Q:

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.

A:

(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.

Q:

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.

A:

(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.

Q:

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

A:

  1. Squeezing of tin container.
  2. Pressing clay by hands.

Q:

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

A:

    Examples of Push:

  1. A suitcase at rest is pushed to move from one place to another. This changes the state of motion of the suitcase.
  2. A football is pushed by a player using his foot.This changes the state of motion of the football.

    Examples of Pull:

  1. To open a drawer, it is pulled out. This changes the state of motion of the drawer.
  2. Rope is pulled to draw water from a well. This changes the state of motion of the water bucket.

Q:

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.

A:

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.

Q:

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?

A:

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.

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