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

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.

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.

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.

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.

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.

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.

A:

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

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.

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.

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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