class 8 science textbook solutions

Force and Pressure – Class 8 Science Textbook Solution

Class 8 Science Chapter 7 Force and Pressure
 
1. Give two examples each of situations in which you push or pull to change the state of motion of objects.
Answer:
Two examples in which push changes the state of motion of objects are:
•    The football player kicking the football.
•    The hockey player hitting the ball will hockey.
Two examples in which pull changes the state of motion of objects are:
•    Pulling out water from the well by pulling rope.
•    Opening of a box in the table by pulling it.
 
2. Give two examples of situations in which applied force causes a change in the shape of an object.
Answer:

•    We can change the shape of a lump of dough by pressing it down with our hands.
•    We can change the shape of the seat of a bicycle by sitting on it.
 
3. Fill in the blanks in the following statements.
1.    To draw water from a well we have to ________ at the rope.
2.    A charged body ________ an uncharged body towards it.
3.    To move a loaded trolley we have to ________ it.
4.    The north pole of a magnet ________ the north pole of another magnet.
Answer:

1.    To draw water from a well we have to pull at the rope.
2.    A charged body attracts an uncharged body towards it.
3.    To move a loaded trolley we have to pull it.
4.    The north pole of a magnet repels the north pole of another magnet.
 
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
1.    To stretch the bow, the archer applies a force that causes a change in its ________
2.    The force applied by the archer to stretch the bow is an example of ________ force.
3.    The type of force responsible for a change in the state of motion of the arrow is an example of a ________ force.
4.    While the arrow moves towards its target, the forces acting on it are due to ________ and that due to ________ of air.
Answer:

1.    To stretch the bow, the archer applies a force that causes a change in its shape.
2.    The force applied by the archer to stretch the bow is an example of muscular force.
3.    The type of force responsible for a change in the state of motion of the arrow is an example of a contact force.
4.    While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction of air.
 
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.
(а) 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:
(а) The agent exerting the force is our muscles (muscular force). The force is exerted on a lemon. The effect of force is that it changes the shape of the lemon and juice is extracted.
(b) The agent exerting the force is our muscles muscular force). The force is exerted on the toothpaste tube. The effect of force is that it changes the shape of the toothpaste tube due to which paste comes out.
(c) The agent exerting the force is the load (non-contact force). The force is exerted on spring and the effect is that the spring expands.
(d) The agent exerting the force is the speed of an athlete. The force is exerted on the body of the athlete. The effect is that he can make his body jump the bar.
 
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:
A blacksmith hammers a hot piece of iron while making a tool. The muscular force applied changes the shape of piece of iron. It flatters and becomes thinner than earlier.
 
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:
The electrostatic force is responsible for the attraction between the balloon and the wall.
 
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 bucket are:
•    The pressure of water contained in it exerted on its wall and on the base of the bucket.
•    Force of gravity by the earth.
The forces acting on the bucket do not bring a change in its state of motion because the net effect of force on it is zero.
 
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:
The two forces that are acting on the rocket immediately after leaving the launching pad are:
•    Force of gravity of earth acting downward, and
•    the frictional force produced by air particles.
 
10. When we press the bulb of a dropper with its nozzle kept in water, the 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) the pressure of water.
(b) the gravity of the earth.
(c) the shape of a rubber bulb.
(d) atmospheric pressure.
Answer:

(d) Atmospheric pressure
Force and Pressure Additional s and Answers
 
1. How can we decide whether an object is moving faster than the other?
Answer:
By finding the speed of both the objects.
 
2. What does the distance move by an object in unit time indicate?
Answer:
The distance moved by an object in unit time is known as speed.
 
3. What makes an object slow Down or go faster, or change its direction of motion?
Answer:
Force makes an object slow down or go faster, or change its direction of motion.
 
4. What do you do to make a football move?
Answer:
We push the football by kicking it.
 
5. What has to do done to move an object?
Answer:
To move an object, it has to be pushed or pulled.
 
6. What is force?
Answer:
A push or a pull on an object is called a force.
 
7. In the figures given below:
(i) Who is pushing whom
(ii) Who is pulling whom.
 

Answer:
•    In fig (a), both the girls appear to push each other i.e., they are applying force on each other.
•    If fig (b) the pair of girls are trying to pull each other i.e. they are applying force on each other.
•    In fig (c) the cow and the man appear to pull each other i.e., they are applying force on each other.
 

8. In the game of tug of wax if two teams pull the rope with equal force then the rope will move in which direction?

Fig. The rope may not move if the two teams pull at it with equal force
Answer:

The rope does not move in any direction.
 

9. When do we add forces?
Answer:
Forces applied on an object in the same direction add to one another.
 
10. Give some examples where force can change the state of motion and direction of an object?
Answer:
Examples, where force can change the state of motion and direction of an object, are:
•    A hockey player in-game constantly changes the state of motion and direction of the ball by hitting it with a hockey stick.
•    In a game of volleyball, players often push the moving ball to their teammates to make a winning move. This changes the state of motion and direction of the ball.
 
11. What effects are produced by force?
Answer:
Force can produce one or more effects:
•    It can bring a stationary object into motion.
•    It can change the speed of moving objects.
•    It can change the direction of a moving object.
•    It can change the shape of an object.
 
12. What is muscular force?
Answer:
The force resulting due to the action of muscles is known as the muscular force.
 
13. Can you push or lift a book lying on a table without touching it?
Answer:
No.
 
14. What is a non-contact force?
Answer:
The non-contact force acts between two objects without being in contact with each other.
 
15. What is magnetic force?
Answer:
The force exerted by a magnet is magnetic force.
 
16. What happens when the same poles of two magnets are brought near each other and what happens when different poles of two magnets are brought near each other.
Answer:
When the same poles of two magnets are brought near each other then there is repulsion when different poles of two magnets are brought near each other then there is attraction.
 
17. The effect of force depends on which things?
Answer:
The effect of a force depends on two things:
•    The amount of force applied the more force applied the greater the effect.
•    The area on which the force is applied the smaller the area the force is applied on, the greater the effect.
 
18. What is the meaning of Pressure?
Answer:
The force on a unit area (perpendicular to the surface) is called pressure.
 
19. I have seen fountains of water coming out of the leaking joints or holes in pipes supplying water. Is it not due to the pressure exerted by water on the walls of the pipes?
Answer:
Yes, this is due to the pressure exerted by water on the walls of the pipes.
 
20. Do gases also exert pressure? Do they also exert pressure on the walls of their containers? Let us find out.
Answer:
Yes, gases exert pressure on the walls of their containers.
 
21. What is the atmosphere?
Answer:
The envelope of air around us is called the atmosphere.
 
22. What is atmospheric pressure?
Answer:
Everything around is being pressed by a column of air. This pressure is called atmospheric pressure.

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