Lakhmir Singh Solutions for Class 8 Science Chapter 11 Force and Pressure - PDF

Very Short Answer Type Questions

1. What is the push or pull on an object known as?

Ans: Force is the push or pull on the object. The motion imparted to objects is due to the action of force.  

Force is defined as an agent that produces motion in a body or changes the state of motion in a body. Force is also applied for lifting an object. Push and pull can also name throwing, hitting, flicking, kicking, picking, lifting, pressing, and shutting. 

2. Why do the shape and size of a balloon change when filled with air or water?

Ans: The air and water matter. Every matter has a characteristic property that has mass and occupies space. When air or water is filled in a balloon, they tend to stretch its rubber boundary and occupy the shape. So, the shape and size of the balloon gets changed when filled with air or water.

3. Name the quantity whose unit is 'newton' \[({\rm{N}})\].

Ans: Force is defined as an agent that produces motion in a body or alters the existing state of the motion in a body.  The unit of the force is called Newton.  One newton is the force that is required to produce an acceleration of \[1m/{s^2}\] in the object of mass. 

4. When a ball is dropped from a height, its speed increases gradually. Name the force which causes this change in speed.

Ans: Due to gravitational force, the speed is changed.  Gravitation is the force of attraction acting on all objects at all times. Gravitation acts on objects which are in contact with the earth or they may be away from the earth. The pull exerted by an object's mass is called gravitational force.

5. What is the unit of force?

Ans: Newton is the unit of force. 

6. Give one example where force changes the shape of an object.

Ans: Two examples of forces that cause a change in the shape of an object are as follows:

1. Squeezing a plastic bottle changes the shape of the bottle.

2. Deformation of clay by pressing it between the hands.

7. Identify the actions involved in the following situations as a push or pull, or both:

1. Opening a drawer.

2. A cricket ball hit by a batsman

3. Drawing a bucket of water from a well.

4. moving a book placed on a table.

5. A football player taking a penalty corner.

6. Moving a wheelbarrow.

Ans: Force is the push or pulls on the object

1. The pull is involved when opening a drawer of a table

2. When a batsman hits a cricket ball, a force is exerted which is a push. 

3. Pull involved when the water is drawing from a well.

4. Both Push and pull are involved when moving a book on the table. 

5. Push is involved when a penalty corner is taken by a football player.

6. Push occurs when the wheelbarrow is moved. 

8. (a) Name two contact forces

(b) Name two non-contact forces.

Ans: 

1. The force is applied through contact. The types of contact forces are: 

• Muscular force

• Frictional force

(b) The source of force and the object on which the force acts remain at the  distance from one another. The non-contact forces are:

• Gravitational Force

• Magnetic Force

• Electrostatic force

9. When a plastic pen is rubbed in dry hair, it attracts tiny pieces of paper. Which force is involved in this process?

Ans: The electrostatic force is involved in this process. The force which induces repulsion of similar charges or attraction of opposite charges is known as electrostatic force. When a plastic pen is rubbed on dry hair, the pen gets electric by friction. When a plastic pen is rubbed on dry hair, it attracts tiny pieces of paper. It is due to electrostatic force.

10. A small device pulls iron nails from a distance. Which type of force is involved in this process?

Ans:  Due to magnetic forces, iron nails are pulled from a distance. 

11. Which force can be used to gather iron pins scattered on the floor?

Ans: Magnetic force can be used to gather pins on the floor. 

12. Name the force which always opposes motion.

Ans: Friction force is always opposing the motion. It always acts in the opposite direction as   an object is moving , and it usually occurs when one surface rubs or drags against another as it moves.

13. Which force makes a rolling ball stop on its own?

Ans: Frictional force makes it stop the rolling ball on its own. The friction occurs between the two surfaces which are in contact with each other.

14.  An inflated balloon was pressed against a wall after it had 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?

Ans: The attraction between the ball and the wall is caused  by an electrostatic force. When we rub the balloon with a synthetic cloth, it becomes charged, and when it has taken it near to the wall, it is attracted to the uncharged wall because of electrostatic force.

15. What name is given to the force acting on a unit area of an object?

Ans: The force acting on a unit area of an object is called pressure. According to the mathematical formula, Pressure= Force/Area. If the area of the object is smaller, the effect of force is greater. 

16. Name the quantity whose one of the units is Pascal (Pa)?

Ans: The unit of pressure is the pascal. 

17. What conclusion do you get from the observation that a fountain of water is created at the leaking joint of pipes of the main water supply line?

Ans: The observation that a fountain of water is created at the leaking joint of pipes of the main water supply line leads us to the conclusion that the pressure exerted on the small hole of the pipe of the main water supply makes the water move out of the pipe through the pressure and thus form a fountain.

18. What type of pressure is involved in the filling of a liquid in a syringe?

Ans: Air pressure is involved in the filling of a liquid into a syringe. When we pull the piston of a syringe out, we create a partial vacuum inside the syringe. This decreases the pressure inside the syringe to a lower pressure than that on its outside and, hence, forces the fluid into the syringe.

19. What substance present in our body balances the atmospheric pressure acting on us?

Ans: Blood in our body balances the atmospheric pressure that acts on it.  Our blood exerts a pressure called blood pressure, which is greater than the atmospheric pressure. 

20. Where will the atmospheric pressure be greater-at ground level or at the top of a high mountain?

Ans: At ground level, atmospheric pressure is at its maximum. When we go to high-altitude places like hills and mountains, the atmospheric pressure decreases. 

21. Name any two devices used in everyday life which work on the existence of atmospheric pressure.

Ans: In our daily life, all the work is due to atmospheric pressure. The devices, such as a drinking straw, a syringe, a dropper, and a rubber sucker, work with the existence of atmospheric pressure. 

• The straw is used for drinking.

• Syringe used in the hospital for draining of blood, and used in an ink pen.

22. If a vacuum is created between two Magdeburg hemispheres joined together, they cannot be separated easily, what presses the hemispheres together?

Ans: The two hemispheres are joined together, and the air is entirely removed from the space between them using a vacuum pump. When all the air is removed from inside the hemisphere, then the hemisphere cannot separate from the pulling by a large force. The hemisphere presses together when the vacuum is created between them because the air is exerted, which creates the force between them and sticks together.

23. What makes a balloon get inflated when air is filled in it?

Ans: When we fill the air in the balloon, pressure is increased inside the balloon and the surface area of the balloon also increases. The force acting by the inside air molecules outwards and the force action by outside air molecules inwards equally. This force makes a balloon get inflated when air is filled in it.

24. Name the substance whose weight produces atmospheric pressure.

Ans: Air is a substance whose weight produces atmospheric pressure. Air has mass and occupies a large space.

25. Where is the pressure greater, 10 m below the surface of the sea or 20 m below the surface of the sea?

Ans: 20 m below the surface of the sea is higher pressure.  A deep-sea driver is provided with a very strong/protective suit because the pressure of water increases as the diver goes deeper and deeper into the sea.

26. What force acting on, an area of \[0.5{\rm{ }}{m^2}\;\]will produce a pressure of \[500{\rm{ }}Pa\]?

Ans: The force acting on a unit area of an object is called pressure. According to the mathematical formula, Pressure= Force/Area. 

From question, 

Area\[ = 0.5{\rm{ }}{m^2}\;\]

Pressure \[ = 500{\rm{ }}Pa\]

\[P{\rm{ }} = {\rm{ }}F/A\]
\[F{\rm{ }} = {\rm{ }}P{\rm{ }} \times {\rm{ }}A\]
\[ = {\rm{ }}500{\rm{ }} \times {\rm{ }}0.5{\rm{ }} = {\rm{ }}250{\rm{ }}N\]

27. Can a liquid exert pressure upwards?

Ans: Yes, the liquid exerts pressure upwards, but it exerts more pressure downwards than upward.  Water molecules are free to move in any direction and exert pressure in all directions, that’s why pressure can exert upwards.

28. Can a liquid exert pressure sideways?

Ans: yes, the liquid pressure can exert in all directions. Water molecules are free to move in any direction, and exert pressure in all directions. The sideways pressure exerted by the liquid also increases with the depth of the liquid. The sideways pressure on the wall is almost zero at its surface. 

29. State whether the following statements are true or false : 

1. The pressure exerted by a liquid depends on the area of the base of its container.

2. A drinking straw works on the pressure exerted by the liquid filled in a soft drink bottle in which it is placed.

Ans: 

1. The given statement is false as the pressure exerted by a liquid in a container depends on its depth. The liquid exerts more pressure as the container's height increases.

2. The given statement is false because the straw does not work on the pressure exerted by the liquid in the soft drink bottle in which it is placed. It works due to the difference in air pressure between inside and outside the drinking bottle when the straw is sucked.

30. Fill in the following blanks with suitable words :

1. To draw water ‘from a well, we have to ______ at the rope.

2. If the two forces applied to an object are equal and act in opposite directions, the net force acting on the object will be _____________

3. Force could be a _________ or a ___________

4. Force has magnitude as well as__________

5. A force arises due to ____________ between two objects.

6. A charged body ___________ an uncharged body towards it.

7. The north pole of a magnet __________ the north pole of another magnet.

8. Force acting on a unit area is called ___________

9. The pressure exerted by a liquid _________ with depth.

10. A drinking straw works on the existence of ___________ pressure.

11. Atmospheric pressure _______________ with increasing height.

Ans: 

1. To draw water from a well, we have to apply pull force to the rope. Force is the push or pull on the object. The motion imparted to objects is due to the action of force. The pull force is applied to it.

2. If the two forces applied to an object are equal and act in opposite directions, the net force acting on the object will be zero. 

3. Force could be a push or a pull. Force is associated with push and pull is resulting in the motion of the body. Force is also applied for lifting an object. Push and pull can also name throwing, hitting, flicking, kicking, picking, lifting, pressing, and shutting. 

4. Force has magnitude as well as direction. Force is a vector quantity. The force depends on the magnitude and direction when we applied. If the magnitude of the applied force or direction of the force changes, it affects the changes in force.

5. A force arises due to interaction between two objects. The man pushing the car is a good example of the interaction of force.

6. A charged body attracts an uncharged body towards it. The force exerted by the electrically charged object is called electrostatic force. 

7. The north pole of a magnet repels the north pole of another magnet. The magnet has two poles, the North and south poles. Like poles of the magnet repel each other, while unlike poles attract each other. The force exerted by the magnet is called magnetic force. 

8. Force acting on a unit area is called pressure. According to the mathematical formula, Pressure= Force/Area. If the area of the object is less, the effect of force is greater. The unit of pressure is the pascal. 

9. The pressure exerted by a liquid increases with depth. The pressure in a liquid is not the same at all depths.

10. A drinking straw works on the existence of atmospheric pressure. The pressure acting on the surface of the soft drink is equal to atmospheric pressure. The increase in atmospheric pressure acting on the surface of the soft drink pushes it up the straw and into our mouth.

11. Atmospheric pressure decreases with increasing height. Atmospheric pressure is maximum at the surface of the earth, when we go to high altitude places like hills, or mountains, then the atmospheric pressure decreases. 

Short Answer Type Questions

31. Define ‘ state of motion ’ of an object. Name the ‘agent’ which can change the state of motion of an object.

Ans: The speed and direction of motion of an object describe its state of motion. At rest, the state of the object is at zero speed. Force is defined as an agent that produces motion in a body or changes the state of motion in a body. 

32. Give two examples of situations where you push or pull to change the state of motion of objects.

Ans: Force is the push or pull on the object. The motion imparted to objects is due to the action of force. Push and pull can also name throwing, hitting, flicking, kicking, picking, lifting, pressing, and shutting. 

The example of push forces are

• To move a heavy box from one room to another, it is pushed. This changes the box's state of motion.

• A football player pushes it with his foot. This changes the ball's state of motion.

33. What is meant by saying that force is due to an interaction’? Give an example to illustrate your answer.

Ans: An interaction between one object to another object results in a force between them. During the interaction, the force increases and makes it move in the applied direction. If there is no interaction between two objects, no force can show its effect. The man pushing the car is a good example of the interaction of force. 

34. In a tug of war; when the two teams are pulling the rope, a stage comes when the rope does not move to either side at all. What can you say about the magnitudes and directions of the forces being applied to the rope by the two teams at this stage?

Ans: If the two forces applied to an object are equal in magnitude and act in opposite directions, then the force acting on the object is zero. Since the force acting on the object is zero, the object does not move, it remains in the same position. A familiar example of equal force acting in the opposite direction is provided by a game called tug of war. A rope is pulled in opposite directions by two teams facing each other. If the forces are applied on both the sides by the team, the force is constant. At a time, one team exerts greater force than another team, and that team will win the tug of war. 

35. What is force? State the various effects of force.

Ans: Force is the push or pull on the object. Force is defined as an agent that produces motion in a body or changes the state of motion in a body. The force cannot be seen. A force can be judged only by the effects and produce the objects on us. 

A force can produce the following effects:

• Force can move a stationary object.

• Force can stop a moving object.

• Force can change the speed of a moving object.

• Force can change the direction of a moving object.

• Force can change the shape and size of the object. 

36. (a) Give one example where force moves a stationary object.

(b) State one example where force stops a moving object.

Ans:    

1. Take a rubber ball and place it on the tabletop. Now gently push the ball along with the surface of the table. The ball at rest begins to move when a force is applied to it. 

2.  In football,   when the goalkeeper dives or jumps to save the goal, he applies force to move the football with his hands. This force applied by the goalkeeper helps in stopping the moving football and saves a goal from being scored. The stopping moving football by a goalkeeper is a force that can stop a moving object.

37. (a) Give one example where force changes the speed of a moving object.

(b) Give one example where force changes the direction of a moving object.

Ans: 

1. We are moving on a bicycle at a certain speed. If someone is pushing the moving cycle from behind, then the speed of the bicycle increases and moves faster. If someone pulls the bicycle from behind, the speed of the bicycle decreases and moves slower. So, the push and pull can change the speed of a moving bicycle. If the force is applied in the direction of motion of the object, its speed increases. 

2. In a cricket match, when a moving cricket ball is hit by a bat, then the direction of the cricket ball changes, and it goes in another direction. The bat's force changes the direction of the moving cricket ball. The change in speed of the object or the direction of its motion or both is called a change in the state of motion. A force can change the state of motion of an object.

38. Why does the shape of an ointment tube change when we squeeze it?

Ans:  The shape of a toothpaste tube changes when we squeeze it because we apply force while squeezing it. The force also affects the change in shape and size of an object. 

39. What happens to the springs of a sofa when we sit on it?

Ans: The shape and size of the spring are changed. Because the forces are acting while we're sitting on the sofa, the force is to change shape and size. When we sit on a sofa with springs, then the springs of the sofa get compressed, and their shape changes. Our weight produces a force on the springs and compresses them.

40. Name the various types of forces.

Ans: There are different types of force that we use and see in our daily life. They are

• Muscular force

• Frictional force

• Magnetic force

• Electrostatic force

• Gravitational force

These types of force, again divided into two groups:

• Contact force

• Non-contact force

Contact force is the force applied only when it's in contact with an object. Contact forces are muscular and frictional forces. 

Non-contact force is the force that exerts on an object without touching it. Gravitational force, magnetic force, and electrostatic forces are non-contact forces.

41. What is muscular force? Give one example of muscular force.

Ans: The force exerted by the muscles of the body is called the muscular body.  Both animals and humans produce muscular force while doing work. Humans produce force by performing various activities, walking, running, jumping, lifting, and so on. Muscular force is always applied directly through the hands.  Cutting vegetables or fruits with a knife is an example of muscular force. 

42. Which of the following are non-contact forces?
Magnetic force, Frictional force, Gravitational force, Muscular force, Electrostatic force.

Ans: A force that can exert by an object on another object from a distance is called non-contact force. There is no physical contact between the objects which produce the force and the object as the force is produced. Gravitational force, magnetic force, and electrostatic forces are non-contact forces.

43. Give two examples from everyday life which show that air exerts pressure.

Ans: In our daily life, some devices work on the existence of atmospheric pressure. The devices such as drinking straws, a syringe, and droppers represent the existence of atmospheric pressure.

Drinking straw: The pressure acting on the surface of the soft drink is equal to atmospheric pressure. The increase in atmospheric pressure acting on the surface of the soft drink pushes it up the straw and into our mouth.

Syringe: A syringe is a glass tube with a nozzle and piston for sucking in and ejecting liquid in a thin stream. The syringe works on the existence of atmospheric pressure. The greater atmospheric pressure acts on the surface of the liquid pushing the liquid into the syringe. 

44. What is a rubber sucker? How does it work? State any one use of a rubber sucker.

Ans:  A rubber sucker is a device made of rubber that sticks to flat and smooth surfaces on pressing. A rubber sucker looks like a small, concave-shaped rubber cup. 

When we press the rubber sucker on a flat, smooth surface, its concave rubber cup gets flattened to a large extent, pushing out most of the air from beneath it. 

When the sucker is pressed against the surface, it squeezes the air on it. A sucker stays attached to a flat surface due to atmospheric pressure. Sucker is usually used to hold objects together with the help of suction.

45. Why do mountaineers usually suffer from nose-bleeding at high altitudes?

Ans: Blood in our body balances the atmospheric pressure that acts on it.  Our blood exerts a pressure called blood pressure, which is greater than the atmospheric pressure. Atmospheric pressure is maximum at the surface of the earth, when we go to high altitude places like hills, or mountains, then the atmospheric pressure decreases. At high altitudes, atmospheric pressure is much lower than blood pressure. Since our blood is at a higher pressure than outside pressure, some of the blood vessels in our blood burst, and noise bleeding takes place at high altitudes. 

46. Describe one activity to show the existence of atmospheric pressure.

Ans: Take a tin can with a tight air cap. When we remove the cap and heat some water in it, the steam comes out, and the inside air goes out. When we can crumble due to air pressure from outside. This activity describes the exertions of atmospheric pressure  . 

47. Explain why water comes out more slowly from an upstairs tap than from a similar tap downstairs.

Ans: The pressure of the water is not the same for all depths. The pressure exerted by the water changes with depth. As the depth of water in the tank increases, the pressure exerted by the water also increases. 

48. What is meant by gravitational force (or force of gravity)? Give it one example.

Ans: Newton said that every object in the universe pulls every other object with a certain force. The pull exerted by an object possessing mass is called gravitational force. The gravitational force between two objects is a force of attraction, and it acts even if the two objects are connected. The gravitational force between the sun and the earth maintains the earth's orbit around the sun. The attractive force of the earth acting upon all the objects is called the force of gravity.

49. Calculate the pressure when a force of 200 N is exerted on an area of:

1. \[10{m^2}\;\]

2. \[5{m^2}\]

Ans: 

1. Given, Force \[F = 200N\]

Using the formula, Pressure =force/Area

Area\[ = 10{m^2}\;\]

\[P = \dfrac{{200}}{{10}} = 20N/{m^2}\]

2. Given, Force \[F = 200N\]

Using the formula, Pressure =force/Area

Area\[ = 5{m^2}\;\]

\[P = \dfrac{{200}}{5} = 40N/{m^2}\]

50. Which force do the animals apply while moving, chewing and doing other activities?

Ans: Muscular force is the force applied by the body's muscles. Animals apply muscular force when moving, chewing, and doing other activities.

51. Which force is responsible for raising our body hair when we try to take off a terylene or polyester shirt in the dry weather?

Ans:  Electrostatic force is responsible for raising our hair when we try to take off a terylene or polyester shirt in dry weather. The force exerted by an electric charge object is called electrostatic force. The electrostatic force can be exerted by a charged object on another object from a distance. It is the non-contact force.  The electrostatic force between any charged object and an uncharged object is always of attraction and two charged objects having like charges is that of repulsion.  

52. Name the type of forces involved in the following:

1. A horse pulling a cart.

2. A sticker attached to steel almirah without glue.

3. A coin falling to the ground on slipping from hand.

4. A plastic comb rubbed in dry hair picking up tiny pieces of paper.

5. A moving boat comes to rest when rowing is stopped.

Ans:

1. The muscular force involved the horses pulling the carts. The animals like a horse, camels, and ox are providing the force of their muscles and help man in doing harder jobs. 

2. Magnetic force is involved, and the sticker is attached to the steel almirah without glue. The magnetic strips help to keep the refrigerator door closed tightly. Certain stickers stick to the iron objects like steel almirahs, refrigerators, etc. without any glue. Such stickers contain tiny magnets which stick to iron objects due to magnetic force.

3. A coin falling to the ground on slipping from the hand involved due to gravitational force. If we drop a coin from some height, it falls towards the earth. Because the earth exerts a gravitational force of attraction on the coin, it pulls down. 

4. A plastic comb rubbed in dry hair picking up tiny pieces of paper involves the electrostatic force. The force in which repulsion of similar charges or attraction of opposite charges is known as electrostatic force. When a plastic pen is rubbed in dry hair and the pen gets electric charges by friction. When this electrically charged pen is brought near the tiny pieces of paper, it attracts them because of electrostatic force

5. A moving boat coming to rest when rowing is stopped involves frictional force. The boat moving in water slows down and finally stops due to the frictional force between the boat and the water. The frictional force exerted by water opposes the motion of the boat and makes it stop.

53. Why does a sharp knife cut objects more effectively than a blunt knife?

Ans: A sharp knife has very thin edges to its blade. A sharp knife cuts objects better because due to its very thin edge, the force of our hand falls over a very small area of an object to produce a large pressure. And this pressure helps to cut the object easily. On the other hand, a blunt knife has a thicker edge. A blunt knife does not cut an object easily because, due to its thick edge, the force on the hand falls over a larger area of the object and produces less force. So, this less pressure cuts the object with difficulty.

54. Explain why wooden (or concrete) sleepers are kept below the railway line.

Ans: Concrete or wooden sleepers are kept below the railway line, because of

• To keep the line equally distant from each other. 

• When the train passes through the railway lines vibration occurs, so this will help to reduce the unwanted vibrations.

• The sleepers have a large surface area; compared with lines, this large surface area decreases the pressure due to the train's weight on the track. 

• The less pressure of the train on the ground, the greater the area of contact, and the railway line may not sink into the ground.

55. Explain why a wide steel belt is provided over the wheels of an army tank.

Ans: A wide steel belt is provided over the wheel of the army tanks. Because they exert less pressure on the ground and do not sink into it. The weight of the tank is heavier. It acts upon the force of gravity that is spread over a larger space giving a small pressure, and it makes it not sink and allows it to move faster because pressure decreases with an increase in area. 

56. Explain why the tip of a sewing needle is sharp.

Ans: The tip of a sewing needle is sharp so that due to its sharp tip, the needle may put force on a very small area of the cloth, producing large pressure sufficient to pierce the cloth being stitched. 

57. Explain why snow shoes stop you from sinking into the snow.

Ans: Large, flat soles of the snowshoes prevent the wearer from sinking into the soft snow and reduce pressure on it. It is easy to walk on the soft snow and if you have flat shoes rather than shoes with small heels. 

58. Explain why, when a person stands on a cushion, the depression is much more than when he lies down on it.

Ans: when a person stands on a cushion, then only his two feet are in contact with the cushion. This weight of the man falls on the same area of the cushion, producing a large pressure. This pressure causes higher depression in the cushion.

On the other hand, when the same man lies on the cushion, the whole body is in contact with the cushion. The weight of the man falls on the large area of the cushion to produce small pressure. And this small pressure is producing less depression in the cushion.

59. Explain why, porters place a thick, round piece of cloth on their heads when they have to carry heavy loads.

Ans: Porters place a thick, round piece of cloth on the head if they have to carry heavy loads of passengers. By using the thick, round piece of cloth, the porters increase the area of contact of the load with the head. Thus, the load now falls on a larger area of the head, the head pressure is reduced, and it is easier to carry the heavy load. 

60. Give one practical application of magnetic force.

Ans. The closing of the door of the refrigerator works on the application of magnetic forces. The magnetic strips help to keep the refrigerator door closed tightly. Certain stickers stick to the iron objects like steel almirahs, refrigerators, etc. without any glue. Such stickers contain tiny magnets which stick to iron objects due to magnetic force.

Long Answer Type Questions

61 (a). What is meant by a contact force? Explain with the help of an example.

Ans: Force that is applied only when it is in contact with an object is called contact force.The examples of contact forces are,

• Muscular force 

• Frictional force

A weightlifter lifting the weight is an example of contact force. This is a muscular force. Weightlifters are using muscular force to lift weights. Another example is the Muscular force of the horse pulling the carts. 

61 (b). What is meant by a non-contact force? Explain with the help of an example.

Ans: The force exerted by an object on another object even from a distance without touching them is called non-contact force. There is no physical contact between the object which exerts the force and the object on which the force is produced.

The examples of non-contact force:

• Magnetic force

• Electrostatic force

• Gravitational force

A magnet attracting a pin made of iron is an example of non-contact force. Another example is when a plastic comb is rubbed in dry, it gets electric charges. 

62 (a). Define frictional force (or friction).

Ans: The force which opposes the motion of one body over another body is called frictional force. When the two surfaces are in contact with each other, frictional force acts on them. 

The factors which affect the frictional force,

• Weight of the moving body on the surface

• The smoothness of surface

62 (b). Explain why frictional force is said to be a contact force.

Ans: The frictional force always acts on all moving objects and is always directed in the opposite direction of motion. The frictional force is an example of contact force since it occurs when the objects of two objects come into contact with each other.

When you stop rowing a boat that is moving in water, it slows down and then stops. The boat moving in water slows and stops due to frictional force between the boat and the water. The frictional force exerted by water opposes the motion of the boat and makes it stop. The two surfaces in contact are the surface of the boat and the surface of the water.

62 (c). Explain why magnetic force is said to be a non-contact force.

Ans: The force exerted by a magnet is called magnetic force. The magnetic force will act even from a distance. The magnets exert a magnetic force on objects made of iron, steel, nickel, and cobalt. Since a magnet exerts its magnetic force on iron objects from a distance, the magnetic force is a non-contact force. 

A magnet exerts a magnetic force on another magnet placed near it. The magnetic forces between two magnets placed in one another can be attractive and repulsive, depending on the poles facing the magnet. 

Due to magnetic forces, iron nails are pulled from a distance. 

63 (a). Define pressure. What is the relation between pressure, force, and area? State the units in which pressure is measured.

Ans: The force acting on a unit area of an object is called pressure. If the area of the object is less, the effect of force is greater. 

According to the mathematical formula, Pressure= Force/Area.  

This formula gives the relation between pressure, force, and area. The SI unit for measuring force is Newton(N) and the unit of measuring area is square meter (\[{m^2}\]). So, the SI unit of pressure is Newtons per square meter(\[N/{m^2}\]) which is also determined in pascal (pa).

Pascal is a very small unit of pressure; so the unit of pressure is also used in kPa (Kilopascal).

63 (b). Explain why school bags are provided with wide straps to carry them.

Ans: A school bag has a wide strap made of thick cloth because the weight of the bag may fall over a large area of the shoulder of the child, to produce less pressure on the shoulder. This low pressure makes it more comfortable to carry a heavy bag. 

If the bag has a strap made of thin string, then the weight of the bag will fall over a small area of the shoulder. This produces high pressure on the shoulder of the child, it becomes very painful to carry the heavy bags.

64 (a). What is meant by atmospheric pressure? What is the cause of atmospheric pressure?

Ans: The air is surrounded by us and has a thick envelope, which is called the atmosphere. The air in our atmosphere extends up to about 300 km above the surface of the earth. The atmosphere contains a high amount of air. This atmosphere exerts a pressure, called atmospheric pressure, on all objects on the surface of the earth. 

In other words, atmospheric pressure is defined as the force exerted on the unit area of a surface by the weight of the surrounding air. Atmospheric pressure is not the same at all places on earth. Atmospheric pressure depends on altitude, when the altitude increases, the atmospheric pressure is decreased.

The atmospheric pressure effects depend on 

• altitude

• Density of air

64 (b). Why are our bodies not crushed by the large pressure exerted by the atmosphere?

Ans: Our body has a liquid called blood, which flows through a blood vessel into every cell of our body.  The atmosphere exerts force on us. This is known as atmospheric pressure. The flowing blood and body fluids inside the body create a pressure that counters the effect of atmospheric pressure, and thus we are not crushed.

64 (c). Explain why atmospheric pressure decreases as we go higher up above the earth’s surface.

Ans: The air molecules at low altitudes are more compressed by gravity than the high altitudes. The atmosphere is maximum on the surface of the earth. The atmospheric pressure on the surface of the earth is 101.3 kilopascals, which is equal to the weight of ten elephants pressed each square meter. The magnitude of the atmospheric pressure is very large. The atmosphere from the earth's surface is high at high altitudes, so the atmospheric pressure is decreased. It is because the weight of the air at high altitudes is decreasing. So, the atmospheric pressure on the top of a high mountain will be less than at the base. 

65 (a). How does the pressure of a liquid depend on its depth? Draw a labelled diagram to show that the pressure of a liquid (say, water) depends on its depth.

(b). Explain why the walls of a dam are thicker near the bottom than at the top.

Ans: 

1. The pressure exerted by a liquid depends on its depth because as the depth increases, so does the pressure exerted by the liquid. This is because the weight of the liquid column pushing the liquid down increases, resulting in increased pressure.

2. The dam's walls are thicker at the bottom than at the top because the pressure exerted by the water increases with depth. The bottom of the dams has thicker walls to withstand this pressure.

Multiple Choice Questions (MCQs)

66. Which of the following is not an example of muscular force?

1. a porter carrying a load on a wheel-barrow.          

2. an apple falling from a tree.

3. a child riding a bicycle.                                             

4. a person drawing water from a well.

Ans: An apple falling from a tree is not an example of muscular force. Muscular force is the force exerted by the body. The apple falls from the tree because of the force of gravity. It is an example of gravitational force.

Hence, option (b) is correct.

67. Which of the following is not an example of the force of gravity? 

1. a leaf falling from a tree. 

2. a boy pushing a cart on a level plane. 

3. a diver jumping into a swimming pool. 

4. a stone falling from the top of a cliff

Ans: A boy pushing a cart on a level plane is not an example of the force of gravity. A force exerts the force of gravity toward the earth. For example, the boy pushes a cart on a level surface because of the push force applied to the cart.

Hence, option (b) is correct.

68. 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: 

1. pressure of water 

2. gravity of the earth 

3. shape of rubber bulb 

4. atmospheric pressure

Ans: The rise of water in the dropper is due to atmospheric pressure. A dropper works on the atmospheric pressure. 

Hence, option (d) is correct.

69. A rectangular wooden block has length, breadth, and height of 50 cm, 25 cm, and 10 cm, respectively. This wooden block is kept on the ground in three different ways, turn by turn. Which of the following is the correct statement about the pressure exerted by this block on the ground?

1. the maximum pressure is exerted when the length and breadth form the base

2. the maximum pressure is exerted when length and height form the base

3. the maximum pressure is exerted when breadth and height form the base

4. the minimum pressure is exerted when length and height form the base

Ans: The maximum pressure is exerted when breadth and height form the base. As we all know, the lesser the surface area, the greater the pressure exerted on it.

Hence, option (c) is correct.

70. Which of the following are contact forces? 

A. Friction 

B. Gravitational force 

C. Magnetic force 

D. Muscular force

1. A and B 

2. B and C 

3. A and D 

4. B and D

Ans: The Friction and muscular force are the contact forces. Contact force is the force applied only when it’s in contact with an object.  Gravitational and magnetic forces are non-contact forces. 

Hence, option (c) is correct.

71. If we release a magnet held in our hand, it falls to the ground. The force responsible for this is: 

1. muscular force 

2. magnetic force 

3. electrostatic force 

4. gravitational force

Ans: If we release a magnet held in our hand, it falls to the ground. The force responsible for this is the gravitational force.

Hence, option (d) is correct. 

72. Which of the following force is utilised in reducing air pollution by removing dust, soot and fly-ash particles from the smoke coming out of chimneys of factories? 

1. magnetic force 

2. gravitational force 

3. electrostatic force 

4. frictional force

Ans: Electrostatic force is used to reduce air pollution by removing dust. The force which induces repulsion of similar charges or attraction of opposite charges is known as electrostatic force.  Electrostatic force removes the charged particles of dust, soot etc. 

Hence, option (c) is correct. 

73. The same force acts on four different objects having the areas given below, one by one. In which case the pressure exerted will be the maximum?

1. \[20{\rm{ }}{m^2}\]

2.  \[50{\rm{ }}{m^2}\]

3. \[10{m^2}\]

4. \[100{m^2}\]

Ans: Pressure exerted will be at its maximum in \[10{m^2}\]. The effects of the force depend on the area of the object on which it acts. If the area is less, the force increases. The force exerted by the unit area is called pressure. 

Hence, option (c) is correct. 

74. Which of the following represent correct values for the normal atmospheric pressure?

A. 101.3 kilopascals 

B. 76 mm of mercury 

C. 101.3 pascals 

D. 76 cm of mercury 

1. A and B 

2. B and C 

3. A and D 

4. B and D

Ans: At sea level, the value for normal atmospheric pressure is 760 mm of a tall mercury column or 101.3 kPa.

Hence, option (c) is correct.

75. Which of the following does not work on the existence of atmospheric pressure?

1. rise of iodine solution in the glass tube of dropper

2. rise of cold drink in a long plastic straw

3. sticking of suction hook on the wall of a room

4. rise of mercury in glass tube of thermometer

Ans: The rise of iodine solution in the glass tube of the dropper does not work on the existence of atmospheric pressure. It is occurring due to the temperature rise.  

Hence, option (a) is correct.

76. The magnitude of force is expressed in the unit of force called : 

1. pascal 

2. kelvin 

3. newton 

4. Magdeburg

Ans: The magnitude of the force is expressed in the unit of force called the newton.

One Newton is the force that can make an object weighing one kilogram move one meter per second. Hence, option (c) is correct.

77. Which of the following changes appreciably when a batsman hits a moving cricket ball? 

A. Shape 

B. Direction 

C. Size 

D. Speed 

1. A and B 

2. B and C 

3. A and C 

4. B and D

Ans: When a batsman hits a moving cricket ball, the ball is likely to change direction and speed. 

Hence, option (d) is correct.

78. Which of the following is not an effect of force?

1. a force can change the speed of a moving object

2. a force can change the direction of a moving object

3. a force can change the composition of a moving object

4. a force can change the shape and size of an object

Ans: A force that can change the composition of a moving object and is not an effect of force. 

A force can produce the following effects:

• Force can move a stationary object.

• Force can stop a moving object.

• Force can change the speed of a moving object.

• Force can change the direction of a moving object.

• Force can change the shape and size of the object. 

Hence, the option (c) is correct.

79. Which of the following is not a non-contact force? 

1. electrostatic force 

2. gravitational force 

3. frictional force 

4. magnetic force

Ans: Frictional force is not a non-contact force. It is a contact force. The force exerted by an object on another object, even from a distance without touching them, is called non-contact force. 

Examples of non-contact force:

• Magnetic force

• Electrostatic force

• Gravitational force

Hence, option (c) is correct.

80. Which of the following scientists gave the idea of the existence of gravitational force?

1. Einstein             

2. James Watt              

3. Faraday             

4. Newton

Ans: Newton gave the idea of the existence of gravitational force by the earth. Newton said that every object in the universe pulls every other object with a certain force. 

Hence, option (d) is correct.

81. Some mustard oil is kept in a beaker. It will exert pressure:

1. downwards only               

2. sideways only             

3. upwards only            

4. in all directions

Ans: It will exert pressure in all directions. Oil is a liquid substance. So, the liquid exerts pressure in all directions. 

Hence, option (d) is correct.

82. A pressure of \[10{\rm{ }}kPa\] acts on an area of \[0.3{\rm{ }}{m^2}.\]The force acting on the area will be: 

1. 3000 N 

2. 30 N

3. 3 N 

4. 300 N 

Ans: The force acting on the area will be 3000 N.

Using the formula,

Pressure=Force/Area

Force=PA
\[ = {\rm{ }}10 \times 1000{\rm{ }} \times 0.3\]

\[ = 3000{\kern 1pt} {\kern 1pt} {\kern 1pt} N\]

Hence, option (a) is correct.

83. The magnitude of atmospheric pressure is equal to the pressure exerted by a :

1. 76 mm tall column of mercury

2. 760 mm tall column of alcohol

3. 76 cm tall column of mercury

4. 760 cm tall column of mercury

Ans: The magnitude of atmospheric pressure is equal to the pressure exerted by a 76 cm tall column of mercury.

Hence, option (c) is correct.

84. The atmospheric pressure is usually measured in the unit of : 

1. newtons per square metre 

2. pascal 

3. cm of mercury 

4. mm of mercury

Ans: The atmospheric pressure is usually measured in the unit of mm of mercury. 

The pressure exerted by the atmosphere due to its weight is referred to as atmospheric pressure. A mercury column in a barometer moves up and down depending on the atmospheric pressure at a given location. It is calculated in millimetres, and the unit used for barometer measurements is mm Hg.

Hence, the option (d) is correct.

85. When a force of 5 N acts on a surface, it produces a pressure of 500 Pa. The area of surface then must be

1. \[10{\rm{ }}c{m^2}\]

2. \[50c{m^2}\]

3. \[100c{m^2}\]

4. \[0.01c{m^2}\]

Ans: The area of the surface then must be \[100c{m^2}\].

Given, Pressure \[ = 500{\rm{ }}Pa\]

Force\[ = 5N\]

Using the formula,

Pressure=Force/Area

Area=Force/Pressure

\[ = 500/5 = 100c{m^2}\]

Hence, the option (c) is correct.

Questions Based on High Order Thinking Skills (HOTS)

86. Two tiny holes are made in a plastic bucket, one near the middle part and the other just above the bottom. When this bucket is filled with water, the water rushes out from the bottom hole much faster than from the upper hole. What conclusion do you get from this observation?

Ans: When two tiny holes in a plastic bucket are made, one near the middle and the other just above the bottom. When you fill this bucket with water, the water rushes out of the bottom hole much faster than it does from the top hole. Because the pressure exerted by the water is also affected by the container's depth. As a result, the hole near the bottom of the bucket is much deeper than the hole near the middle of the bucket. As a result, the water is pushed out with more pressure from the hole near the bottom.

87. What is common in the working of the devices such as a drinking straw, a syringe, a dropper and a rubber sucker?

Ans: In our daily life we work on the existence of atmospheric pressure. All the devices, such as the drinking straw, syringe, dropper, and rubber sucker, depend on atmospheric pressure to work.

88. 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 (Ignore the frictional force due to air resistance)

Ans: Gravitational force and frictional force are the two forces acting on the rocket immediately after it departs the launch pad. Gravitational force acts downwards towards the earth, while frictional force acts upwards due to the surrounding air.

89. One student says that water exerts pressure on the bottom of the bucket but another student says that water exerts pressure on the sides of the bucket. What would you like to say?

Ans: None of the students said the right statement. Water, oil, alcohol, and petrol are the liquid substances used in our daily life. All the liquid exerts pressure on the base and wall of the containers. The pressure exerted by the water is equivalent in all directions because the force is equal on all sides of the bucket. So, the water is exerted in all directions of the bucket.

90. 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.

Ans: The two types of force acting on the plastic bucket containing water held above ground level in hand are:

• The muscular force applied by hand

• The gravitational force applied by the earth.

These two forces are equal and opposite to each other, so they do not cause any change in the state of motion.

Importance of Lakhmir Singh Class 8 Science Chapter 11 Force and Pressure

Chapter 11 of Class 8 Science explains what force is and how it is produced. This physical quantity is described along with the derivation of units to measure. It also explains how force is responsible for the change in the shapes and state of motion.

This chapter explains the different kinds of forces we observe, such as push and pull. It also explains what contact and non-contact forces are, along with descriptions and examples. Students will also come to know about the magnetic and gravitational forces of attraction in this chapter.

They will also learn about friction and how this force resists motion. While explaining such forces, this chapter will explain what a vector and scalar quantity are. It will also explain why force is a vector quantity. Students will get proper assistance from the Force and Pressure Class 8 Activities Solutions to find the right answers and develop their concepts related to this chapter.

This chapter will explain what states of motion are and how they can be changed by applying force. It will also describe what pressure is and how applying force can create pressure on a surface.

Hence, this chapter develops the conceptual foundation related to mechanics, a crucial part of Physics.

Benefits of Lakhmir Singh Class 8 Science Chapter Force and Pressure Notes And Solutions

Chapter 11 of Class 8 Science explains what force is and how it is produced. This physical quantity is described along with the derivation of units to measure. It also explains how force is responsible for the change in the shapes and state of motion.

This chapter explains the different kinds of forces we observe, such as push and pull. It also explains what contact and non-contact forces are, along with descriptions and examples. Students will also come to know about the magnetic and gravitational forces of attraction in this chapter.

They will also learn about friction and how this force resists motion. While explaining such forces, this chapter will explain what a vector and scalar quantity are. It will also explain why force is a vector quantity. Students will get proper assistance from the Force and Pressure Class 8 Activities Solutions to find the right answers and develop their concepts related to this chapter.

This chapter will explain what states of motion are and how they can be changed by applying force. It will also describe what pressure is and how applying force can create pressure on a surface.

Hence, this chapter develops the conceptual foundation related to mechanics, a crucial part of Physics.

Benefits of Lakhmir Singh Class 8 Science Chapter Force and Pressure Notes And Solutions

• These solutions will offer exceptional study support to the students during revising and practising the exercise questions. They will find the simpler explanation of the entire chapter easier to understand and recall during an exam.

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• Students will also be able to resolve doubts using these solutions on their own. They can access these solutions at their convenience and make the study sessions more productive.

Download Force and Pressure Class 8 Chapter 11 Lakhmir Singh Solutions PDF

Why wait then? Get the free Class 8 Force and Pressure notes PDF today and complete your study material for force and pressure. Prepare this fundamental chapter well by using the notes and exercise solutions formulated by the experts. Focus on how the experts have answered the fundamental questions and learn how to apply the concepts perfectly. This way, you can score more and stay ahead of the competition.