NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure

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Updated at 2021-03-25


NCERT solutions for class 8 Science Chapter 11 Force And Pressure Exercise

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



NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure


Answer :

1. Push situation

(a) Drawer is closed by pushing.

(b) Wooden box is moved forward by pushing.

2. Pull situation

(a) Water is drawn from the well by pulling the rope attached to a container.

(b) A cart is pulled by the horse to bring the cart into motion.

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



NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure


Answer :

The forces which can change the shape of an object are as follows:

i) When the clay is pressed between the hands, it changes its shape on the application of a muscular force and gets deformed.

ii) When the rubber band is stretched, it changes its shape on the application of a muscular force and thus gets stretched until released.

Q.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.



NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure


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 pull or push it.

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

EXPLANATION

1) Pull is a force which helps to counteract the gravitational force of the earth. Water can be pulled upwards when the weight of water drawn is less than the applied pull by the individual. That means if the force with which the earth pulls the same amount of water( weight of water drawn) by means of gravitational force in the downward direction, is less than the muscular force with which an individual pulls the same amount of water upwards, then only the water can be pulled out.

2) A charged body attracts an uncharged body by oppositely charging it. That means, if the charged body is positively charged( lose of electrons) then it converts the uncharged body into negatively charged body and then undergoes attraction. And if the charged body is negatively charged(gain of electrons) then it converts the uncharged body into positively charged body and then undergoes attraction.

3) To move a loaded trolley, we have to overcome the frictional force of the ground by applying a muscular force of magnitude greater than the magnitude of the frictional force. This muscular force can be a push or a pull.

4) As we know that like poles repel each other while unlike poles attract each other. Hence north pole of one magnet repels the north pole of the other magnet.

Q.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.



NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure


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.

EXPLANATION

When the archer stretches the bow, then he applies muscular force on the bow using his hands and thus stretches the bow by changing its shape.

Now there is a change in state of the arrow which was in contact with the bow. As a result it proves that some force is exerted on the arrow that had brought change in its state of rest, and this force is called as contact force.

When the arrow moves forward, as it moves above the ground level, hence the earth exerts a downward force on it due to gravity. And as it moves through the air, the air opposes its motion by resisting its motion in the forward direction because of the friction of air.

Q.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.



NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure


Answer :

(a) While squeezing a piece of lemon, we use our muscular force using our fingers, to extract its juice. This muscular force acts on the lemon. As a result, the shape of the lemon gets changed.

(b) While taking out paste from a toothpaste tube, we use our muscular force using our fingers. This muscular force acts on the toothpaste tube. As a result, the shape of the tube gets changed.

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

(d) The muscles of the athlete exert muscular force on his body to overcome the frictional force exerted by the ground, as a result of which there is an effect of the force that changes the state of motion of the athlete in the forward direction.

Q.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?



NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure


Answer :

A blacksmith hammers a hot piece of iron while making a tool, because iron is a metal and has the property of malleability and ductility due to which it can be beaten while heating to get the required shape.

The force due to hammering causes the change in the shape of the iron and thus iron can be moulded in the shape of the required tool.

Q.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?



NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure


Answer :

When an inflated balloon was rubbed with a piece of synthetic cloth, then the synthetic cloth loses electrons to the balloon, thus becoming positively charged and making the balloon negatively charged.

Now when the balloon is pressed against the wall, then the negatively charged balloon makes the uncharged wall to lose electrons away from the balloon, thus making the wall positively charged.

It was found that the balloon sticks to the wall because of this electrostatic attraction in which both the charged particles tend to attract each other, thus getting stuck with each other.

Q.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.



NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure


Answer :

When we lift the plastic bucket containing water just above ground level, then the gravitational force starts acting upon the plastic bucket filled with water in the downward direction.

The magnitude of gravitational force of earth acting in the downward direction is equal to the weight of the plastic bucket filled with water. As the muscular force is equal in magnitude and opposite in direction to the magnitude of gravitational force, hence it counteracts with the gravitational pull of the earth, thus not letting the bucket fall down. The two forces are equal in magnitude and opposite in direction. Therefore, the net force on the bucket is zero. Hence, there is no change in its state of motion.

But if the plastic bucket filled with water is lifted up and is not provided with any external force to counteract the gravitational force, then the net force would have been towards the downward direction, and thus the bucket would have been fallen down.

Q.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.



NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure


Answer :

If a rocket is fired upwards to launch a satellite in its orbit, then the two forces acting on the rocket immediately after leaving the launching pad are gravitational force of the earth that acts in the downward direction and frictional force of air that acts in an opposite direction to the motion of the rocket.

Gravitational force of earth is a force that is exerted by the earth on all objects present in it. When the rocket starts moving in the upward direction, Earth exerts a force which is equal to the weight of the rocket in the downward direction.

Frictional force of air is the force resisting the relative motion of any object that moves in air. When the rocket starts moving in the upward direction, the air exerts an equal and opposite force on the rocket, thus resisting its motion.

Q.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



NCERT Solutions for Class 8 Science Chapter 11 Force And Pressure


Answer :

Atmospheric pressure.

EXPLANATION

When we press the bulb of a dropper with its nozzle kept in water, air in the dropper escapes in the form of bubbles. As all the air is removed by pressing the bulb of dropper, so the pressure of air inside the bulb decreases. The atmospheric pressure outside the bulb of the dropper i.e., pressure of air in the water becomes higher than the inside pressure of the bulb. So the air in the water flows from high pressure region to low pressure region i.e., towards the inside of the bulb. As a result there is rise of water in the dropper.



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