Class 8 - Physics - Force and Pressure

Question1.

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

Two examples when we push the objects:-

1. Push a ball.

Explanation:-

When we push a ball it changes from its state of rest to motion.

2.   Push a door.

Explanation:-

Earlier door was closed when we push the door it opens.

Two examples when we pull the objects:-

1. Pull a trolley.

Explanation:-

When we pull a trolley it changes its state of rest to motion

2. Pull a door.

Explanation:-

When we pull a door, it either gets opened or closed.

Question2.

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

1. Press a balloon.

Explanation:-

When we press an inflated balloon with both our hands, the balloon gets squeezed and changes its shape from spherical to oval.

2. Squeeze a toothpaste tube.

Explanation:-

When a toothpaste tube is squeezed toothpaste comes out and also tube keeps on changing its shape.

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 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 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) 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 contact force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a muscular 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.

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.

(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.

• Squeezing a piece of lemon between the fingers to extract its juice.

Agent exerting force: - Fingers

Object: - Lemon

Effect: - Change in Shape.

• Taking out paste from a toothpaste tube.
• Agent exerting force: - Fingers

Object: - Toothpaste tube

Effect: - Change in Shape.

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

Object: - Spring

Effect: - Change in Shape of spring.

• An athlete making a high jump to clear the bar at a certain height.
• Agent exerting force: - Muscles

Object: - Athlete

Effect: Change in Shape of motion.

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?

Hammering applies force on hot iron which changes it to a desired shape.

When the iron is hot it is very flexible enough to take any shape.

When we are hammering it a force is being applied this result in its change in 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?

The force which is responsible for the attraction between the ball and the wall is electrostatic force.

When we rub the balloon by a synthetic cloth, it gets charged. When the balloon is taken near the wall,

it will get attracted towards the uncharged wall because of the electrostatic force.

Electrostatic force is the force exerted by a charged body on another uncharged body.

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?

The forces acting on a plastic bucket are:

1. Gravitational force: It is acting downwards.
2. Muscular force: It is applied by our hands to lift the bucket in upward direction.

Although these forces are acting on the bucket but no change is found in its state of motion

because the two forces are balancing each other and as a result net force is zero.

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.

Two forces acting on the rocket immediately after leaving the launching pad are:

1. Atmospheric force.
2. Gravitational in downward 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