class 8 ch 11

Chapter 11. Force And Pressure | Science | CBSE Class VIII (8th) | Solved Exercises

Question1. : Give two examples each of situations in which you push or pull to change the state of motion of objects.
Answer :

1. Opening or shutting a door
2. A cricket ball hit by a batsman  

 

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

1. Rolling a ball of flour dough for making a chapati at home
2. Bursting of an inflated balloon by pressing between the palms.

Question 3. Fill in the blanks in the following statements:
(a) To draw water from a well we have to apply the force of pull at the rope.
(b) A charged body attracts or pulls  an uncharged body towards it.
(c) To move a loaded trolley we have to either pull it or push it.
(d) The north pole of a magnet repels  the north pole of another magnet.

Question 3. 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 shape.
(b) The force applied by the archer to stretch the bow is an example of muscularforce.
(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 gravityand 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.

 

Answer : 

Situation	Agent	Object	Effect
(a) Squeezing a piece of lemon between the fingers to extract its juice.	Muscular Force of Fingers	Lemon Outer Surface	Shape of the piece of lemon gets changed and juice gets extracted.
(b) Taking out paste from a toothpaste tube.	Muscular Force of Fingers	Wall of Tube and Paste	Shape of the tube gets changed on pressing  and paste is squeezed out.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.	Weight of load by Force of Gravity	Spring	As the other end of the spring is fixed to wall, so the spring will expand downward.
(d) An athlete making a high jump to clear the bar at a certain height.	Muscular Force	Weight of athlete (Force of Gravity)	The athlete is able to lift his weight to jump across the bar at certain height

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?

Answer : When  A blacksmith hammers a hot piece of iron while making a tool, its shape gets changed into a desired shape of tool.

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? 

Answer : When we rub, an inflated balloon with a piece of synthetic cloth, its surface gets charged due to electrostatic force. On pressing it against a wall, the charged surface of balloon, due to electrostatic force, attracts uncharged surface of the wall and the balloon sticks to the wall.

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.
Answer : The forces acting on the plastic bucket containing water, held above the Ground Level are : 

1. Muscular Force which is being exerted to pull (lift) the weight of filled bucket, caused by force of gravity
2. The force of Gravity, in the form of weight of bucket, acting in downward and opposite direction of muscular force

The forces acting on the bucket do not change its state of motion, as muscular force causing upward pull is cancelled out by downward pull of gravity.


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.
Answer : When a rocket is fired upwards to launch a satellite,two forces acting on the rocket immediately after leaving the launching pad are : 

1. Combined weight of the rocket and payload ( Satellite), caused by the force of gravity, acting in the downward direction, towards the center of the earth
2. The force of thrust, caused by the burning of propellant fuel in the rocket. The force of thrust which exceeds force of gravity, acts in the opposite direction of gravity, and makes the rocket move upward 

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

Answer : (c) shape of rubber bulb and (d) atmospheric pressure
On releasing the pressure on the bulb of dropper, it tries to regain its original form with greater volume earlier, inside. As the opening of droppertube is dipped in water, water rises up to fill empty inside of dropper due to atmosheric pressure on the outer surface of water.

CBSE Class VIII (8th) | Science | Chapter 11. Force And Pressure |  |  Solved Exercises