Class 8 Science Chapter 11 Force and Pressure

NCERT Solutions for Class 8 Science Chapter 11 Force and Pressure

Topics and Sub Topics in Class 8 Science Chapter 11 Force and Pressure:

Section Name	Topic Name
11	Force and Pressure
11.1	Force – A Push or a Pull
11.2	Forces are due to an Interaction
11.3	Exploring Forces
11.4	A Force can Change the State of Motion
11.5	Force can Change the Shape of an Object
11.6	Contact Forces Muscular Force
11.7	Non-contact Forces Magnetic Force
11.8	Pressure
11.9	Pressure Exerted by Liquids and Gases
11.10	Atmospheric Pressure

Force and Pressure Class 8 Science NCERT Textbook Questions

Question 1.
Give two examples each of the situations in which you push or pull to change the state of motion of objects.
Answer:
(i) Push: We close drawer by pushing.
We move a wooden box by pushing.
(ii) Pull: We draw water from a well by pulling the rope.
A horse pulls a cart.

Question 2.
Give two examples of situations in which applied force causes a change in the shape of an object.
Answer:
When we apply force on a rubber band to stretch it and on clay to change 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.
Answer:
(a) pull
(b) attracts
(c) push
(d) repels

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.
Answer:
(a) shape
(b) muscular
(c) contact
(d) gravity, friction

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:
(a) Agents are fingers, object is lemon, effect of force changes the shape of lemon.
(b) Agents are fingers of the person squeezing the tube, object is toothpaste tube and effect of the force can be observed as the paste coming out of the tube (change in shape).
(c) Agent is the load suspended, object is the spring and effort can be seen in the form of elongation of spring on suspension of load (change in shape).
(d) Agent is muscles of athlete, object is athlete himself and effect of the force changes the state of motion of the athlete.

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

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:
Forces acting on bucket are as follows:
(i) Muscular force of arms acting upward.
(ii) Force of gravity acting downward.
Both the forces do not bring any change in the state of motion because both of them are acting in equal and opposite directions and thus they cancel each other’s effect.

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:
The forces that act when a rocket leaves launching pad are as follows:
(i) Gravitational force of the earth (downward)
(ii) Frictional force of air (in opposite 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
Answer:
(d) atmospheric pressure

Force and Pressure Class 8 Science NCERT Intext Activities Solved

Activity 1 (NCERT Textbook, Page 128)
Table 11.1 gives some examples of familiar situations involving the motion of objects. You can add more such situations or replace those given here. Try to identify action involved in each case as a push and/or a pull and record your observations. One example has been given to help you.
Identifying Actions as Push or Pull

Activity 2 (NCERT Textbook, Page 130)
Choose a heavy object like a table or a box, which you can move only by pushing hard. Try to push it all by yourself. Can you move it? Now ask one of your friends to help you in pushing it in the same direction [Fig. 11.1(a)]. Is it easier to move it now? Can you explain why? Now push the same object, but ask your friend to push it from the op¬posite side [Fig. 11.1 (b)]. Does the object move? If it does, note the direction in which it moves. Can you guess which one of you is applying a larger force?

Solution:
(i) The box moves with difficulty when we push alone but moves easily if helped by our friend. Thus, force applied on an object in the same direction add to one another.
(ii) When the object is pushed from the opposite side, it does not move. Thus, if two forces acting in the opposite direction on an object the net force acting on it is the difference between the two forces.

Activity 3 (NCERT Textbook, Page 131)
Jake a rubber ball and place it on a level surface such as a tabletop or a concrete floor. Now, gently push the ball along the level surface (Fig. 11.2). Does the ball begin to move? Push the ball again while it is still moving. Is there any change in its speed? Does it increase or decrease?
Next, place your palm in front of the moving ball. Remove your palm as soon as the moving ball touches it. Does your palm apply a force on the ball? What happens to the speed of the ball now? Does it increase or decrease? What would happen if you let your palm hold the moving ball?

Solution:
When we push the ball along the level surface, the ball begin to move. When we push the ball again while it is still moving, increases its speed.
When we place our palm in front of the moving ball, decreases its speed. Hence, force can change the state of motion.

Activity 4 (NCERT Textbook, Page 132)
Take a ball and place it on a level surface as you did in Activity 11.3. Make the ball move by giving it a push. Now place the ruler from your geometry box in its path as shown in Fig. 11.3. In doing so, you would apply a force on the moving ball. Does the ball continue to move in the same direction after it strikes the ruler? Repeat the activity and try to obstruct the moving ball by placing the ruler such that it makes different angles to its path. In each case note your observations about the direction of motion of the ball after it strikes the ruler.
Solution:
Ball changes direction after it strikes the ruler. Thus, a force can change direction of motion of a moving object.

(a) A ball set in motion by pushing it along a level surface and
(b) the direction of motion of the ball after it strikes the ruler placed in its path

Activity 5 (NCERT Textbook, Page 133)
Some situations have been given in Column 1 of Table 11.2 in which objects are not free to move. Column 2 of the Table suggests the manner in which a force can be applied to each object while Column 3 shows a diagram of the action. Try to observe the effect of the force in as many situations as possible. You can also add similar situations using available material from your environment. Note your observations in Columns 4 and 5 of the table.
Studying the Effect of Force on Objects

Solution:
From the above actions, we can conclude that force can change the shape of an object.

Activity 6 (NCERT Textbook, Page 135)
Take a pair of bar magnets. Place the longer side of one of the magnets over three round shaped pencils or wooden rollers as shown in Fig. 11.4. Now bring one end of the other magnet near the end of the magnet placed on the rollers. Make sure that the two magnets do not touch each other. Observe what happens. Next, bring the other end of the magnet near the same end of the magnet placed on the rollers (Fig. 11.4). Note what happens to the magnet placed on the rollers every time another magnet is brought near it.

Solution:
When unlike poles of the magnet are brought near the other magnet, they attract each other and moves in the direction of another magnet. When like poles of a magnet are brought near to the other magnet, they repel each other and moves in the direction away from the other magnet.
Thus, the force exerted by a magnet on another magnet is a type of non-contact force.

Activity 7 (NCERT Textbook, Page 136)
Take a plastic straw and cut it into nearly two equal pieces. Suspend one of the pieces from the edge of a table with the help of a piece of thread (Fig. 11.5). Now hold the other piece of straw in your hand and rub its free end with a sheet of paper. Bring the rubbed end of the straw near the suspended straw. Make sure that the two pieces do not touch each other. What do you observe?
Next, rub the free end of the suspended piece of straw with a sheet of paper. Again, bring the piece of straw that was rubbed earlier with paper near the free end of the suspended straw. What do you observe now?

Solution:
A straw rubbed with paper attracts another straw but repels it if it has also been rubbed with a sheet of paper.The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.

Activity 8 (NCERT Textbook, Page 138)
Take a transparent glass tube or a plastic pipe. The length of the pipe/tube should be about 15 cm and its diameter should be 5-7.5 cm. Also take a piece of thin sheet of a good quality rubber, say, a rubber balloon. Stretch the rubber sheet tightly over one end of the pipe. Hold the pipe at the middle, keeping it in a vertical position (Fig. 11.6). Ask one of your friends to pour some water in the pipe. Does the rubber sheet bulge out? Note also the height of the water column in the pipe. Pour some more water. Observe again the bulge in the rubber sheet and the height of the water column in the pipe. Repeat this process a few more times. Can you see any relation between the amount of the bulge in the rubber sheet and the height of the water column in the pipe?

Solution:
The amount of bulge in the rubber sheet increases with the height of the water column in the pipe. Thus, the pressure exerted by water at the bottom of the container depends upon the height of its column.

Activity 9 (NCERT Textbook, Page 138)
Take a plastic bottle. You can take a discarded water or soft drink bottle. Fix a cylindrical glass tube, a few cm long near its bottom as shown in Fig. 11.7. You can do so by slightly heating one end of the glass tube and then quickly inserting it near the bottom of the bottle. Make sure that the water does not leak from the joint. If there is any leakage, seal it with molten wax. Cover the mouth of the glass tube with a thin rubber sheet as you did in Activity 11.8. Now fill the bottle upto half with water. What do you observe? Why does the rubber sheet fixed to the glass tube bulge this time? Pour some more water in the bottle. Is there any change in the bulge’of the rubber sheet?

Solution:
Yes, the rubber tube bulge out and the bulging increases with the amount of water in the bottle. Thus, liquid exert pressure on the walls of the container.

Activity 10 (NCERT Textbook, Page 139)
Take an empty plastic bottle or a cylindrical container. You can take a used tin of talcum powder or a plastic bottle. Drill four holes all around near the bottom of the bottle. Make sure that the holes are at the same height from the bottom (Fig. 11.8). Now fill the bottle with water. What do you observe?
Do the different streams of water coming out of the holes fall at the same distance from the bottle? What does this indicate?

Solution:
We observed that different streams of a waterfall at the same distance from the bottle. Thus, it indicates that liquids exert equal pressure at the same depth.

Activity 11 (NCERT Textbook, page 140)
Take a good quality rubber sucker. It looks like a small rubber cup (Fig. 11.9). Press it hard on a smooth plane surface. Does it stick to the surface? Now try to pull it off the surface. Can you do it?

Solution:
when we press the sucker, most of the air between its cup and the surface escapes out. The sucker sticks to the surface because the pressure of the atmosphere acts on it. To pull the sucker off the surface, the applied force should be large enough to overcome the atmospheric pressure. Thus, this activity gives us an idea about the magnitude of atmospheric pressure.

NCERT Solutions for Class 8 Science Chapter 11 – 1 Mark Questions and Answers

Question 1.
When we press the bulb of a dropper with its nozzle kept in water, air in the dropper seems 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 [NCERT]

· pressure of water

· gravity of the earth.

· shape of rubber bulb.

· atmospheric pressure.

Answer:
atmospheric pressure

Question 2.
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]
Answer:
Electrostatic force.

Class 8: Force and Pressure

B. V. Short types:

1. True. [Push or pull acting on a body are called force. The direction in which an object is pushed or pulled is called the direction of the force.]

2. False. [When we apply the brakes to slow down a bicycle it will bicycle will move forward. Thus, the bicycle will move in the opposite direction of the applied force.]

3. The net force acting on the object is A + B. [If two forces act on a body in the same direction, then the net force acting on object will be sum of the two forces.]

4. The net force acting on the object is A B.[If two forces are applied on an object in opposite directions, then the net force acting on the object is the difference between the two forces. ]

5. No. The net force of all forces determines the acceleration of the body

6. False. [Every object in the universe exerts gravitational force on other matter.]

7. Weight [An object’s weight is a measure of the Earth’s gravitational pull acting on it. Weight   = mg]

8. Magnetic force          9. Electrostatic force          10. Friction force

11. The larger the area over which a force acts, the lesser the pressure is.

12. Pressure exerted by a liquid increases with depth and acts in all directions.

13. True. [Due to the atmospheric pressure of air, we experience a force equal to that exerted by a mass of 1 kg on every square centimetre of our body.]

14. False. Atmospheric pressure is caused by the weight of air molecules above the atmosphere. As we go to higher altitudes, the air becomes thinner and the atmospheric pressure decreases

C. Short type (i)

1. Force can be described by two things: its magnitude and the direction in which it acts. Force is a vector quantity.

2. If several forces act on a body in different directions, the effect on the body will be due to the magnitude and direction of the net force acting on it. So, the body will move in the direction of the resultant force

3. Gravitational force is the force with which the Earth pulls everything towards itself. So, if an object is thrown up, it slows down and then comes down because of the gravitational force of the Earth.

4. Mass
The mass of a body is the quantity of matter contained in it
It is a scalar quantity that has only magnitude, no direction
The SI unit of mass is the kilogram (kg).

Weight
The weight of a body is the force with which it is attracted towards the centre of the Earth
Weight is a vector quantity having magnitude as well as direction
The SI unit of weight is the Newton (N).

5. Every magnetic substance is surrounded by its own invisible magnetic field, depending on its pole strength. A magnet can act from a distance because of the field of magnetic force.

6. Pressure is the trust acting on a unit area of an object. Pressure = Thrust/Area
Pressure depends on two factors: (i) Force applied (ii) Area over which the force acts

7. Atmospheric pressure is the force per unit area exerted on the surface of the Earth by the atmosphere. Atmospheric pressure is caused by the weight of air molecules in the atmosphere.
8. It is difficult to cut vegetables with a blunt knife because it has more area compared to a sharp knife, therefore the pressure exerted by the blunt knife is less than that of the sharp knife.

D. Short type (ii)

1. Force: Force is an agent that change or try to change the state of an object. The SI unit of force is kg m/s² or Newton (N).
Effects of force: 1. Change the state of rest and motion. 2. It can change the speed of a moving object. 3. It can change the direction of a moving object. 4. It can change the shape or size of an object.

2. Contact Force: The force which acts by touching objects is called Contact Force. For example muscular force, friction force
Non Contact Force: The force which act without touching objects is called non contact Force .. For example: Gravitational force, Electrostatic force

3. Friction is a kind force that always opposes motion of body. For example, when you stop pedalling a bicycle, it slows down and stops after some time because of the frictional force.

4. Caterpillar tracks are used in battle tanks instead because it distributes force over larger area and reduce the pressure over surface. This prevents tracks from sinking in areas where wheeled vehicles of the same weight can easily sink.

5 The pressure exerted by the liquid is given by the following expression:
P = ρgh
Here, ρ = Density of the liquid g = Acceleration due to gravity h = Height/depth of the liquid column
Here, pressure is directly proportional to the height of the liquid column. If we go down into the liquid, the depth (h) of the liquid column increases. Thus, the pressure in the liquid increases with increasing depth.

6. When we press a rubber sucker on a smooth surface, we remove some of the air between the sucker and the smooth surface. This deficiency of air lets the air outside to exert pressure on the surface of the sucker. Force due to the pressure keeps the sucker stuck to the surface.

HOTs:
1. False. If no external force is applied on the body, then there must be two equal and opposite forces acting on the body. Both these forces add up to zero.

2. False. Contact force is the force in which a direct contact is required between two bodies. The contact may also be with the help of a stick or a piece of rope. If we use a stick to push an object, we make the contact with the help of stick. This push or force is caused by the action of the muscles in our body. This muscular force is a contact force.

3. The magnitude of gravitational force depends on the masses of two objects and the distance between them.
Gravitational force generally felt between objects of greater masses. Since mass of building and our body smaller as compared to the earth, we are not pulled towards the building.

4. It is necessary to define pressure as a separate quantity to find the impact of force on a surface.
As we know that the impact of force depends on both the amount of force applied and the area over which it is applied. So, only force is not enough for this purpose.

5. If one of the windows in a jet plane breaks due to an accident, the air pressure inside   plane decreases. At such low pressure, we can have problems like nose bleeding, because the pressure exerted by the blood in our body will be much higher than the pressure outside. This can cause blood vessels to burst.

6. If we press a rubber sucker on a rough surface, it will not stick to the surface because an airtight seal cannot be achieved between the surfaces. It is so because air rushes into the space between the rubber sucker and the rough surface.