Force and Pressure Class 8 Science Extra Questions and Answers

Force and Pressure Class 8 Science Chapter 11 Extra Questions and Answers are provided here. We prepared these extra questions based on the latest NCERT Class 8 Science Book. CBSE Class 8 Science Chapter 11 Force and Pressure Extra Questions will help you to properly understand a particular concept of the chapter.

Class 8 Science Chapter 11 Force and Pressure Extra Questions

Very Short Answer Type Question

Question 1: What is the unit of force?

Answer: Newton

Question 2: Give one example where force moves a stationary object.

Answer: Pushing a chair

Question 3: Give one example where force stops a moving object.

Answer: A fielder stopping a ball

Question 4: What is force?

Answer: A push or a pull on an object is called a force.

Question 5: Does liquid exert pressure in all directions?

Answer: Yes, Liquid exerts pressure in all directions.

Question 6: Why does the shape of tooth paste tube change when we squeeze it?

Answer: The shape changes because our finger applies force on the tube.

Question 7: What can bring change in state of motion of an object?

Answer: A force may bring a change in the state of motion of an object.

Question 8: Which force can be used to gather iron pins scattered on the floor?

Answer: Magnetic force can be used to gather iron pins scattered on the floor.

Question 9: Which force do the animals apply while moving, chewing and doing other activities?

Answer: Muscular force

Question 10: Which force is responsible for raising our body hair when we try to take off a terylene or polyester shirt in the dry weather?

Answer: Electrostatic force

Question 11: Write one example where force changes the speed of a moving object.

Answer: Speed of moving a rubber tyre or a ring increases whenever it is pushed.

Question 12: Why does a boat come to rest when we stop rowing it?

Answer: Friction between water and the boat brings it to a stop once we stop rowing.

Question 13: Write any two application of atmospheric pressure in our everyday life.

Answer: Drinking straw and Dropper work on the existence of atmospheric pressure.

Question 14: What is pressure?

Answer: The force acting on a unit area of a surface is called pressure.
Pressure = force / area on which it acts

Question 15: Give one practical application of magnetic force.

Answer: The closing of door of refrigerator works on the application of magnetic forces.

Question 16: Give one example from daily life where force changes the shape of an object.

Answer: Example: The shape of dough changes on pressing with a rolling pin to make chapatis

Short Answer Type Questions

Question 1: Explain why, snow shoes stop you from sinking into snow.

Answer: The snow shoes have large, flat soles so they exert less on the soft snow and stop the wearer from sinking into it.

Question 2: What happens to the springs fixed to the seat of a bicycle when we sit on it?

Answer: When we sit on the seat of a bicycle, the force exerted by the body weight compresses the spring and changes its shape.

Question 3: Why do balloons expand when filled with air?
Or
Why does the balloon get inflated?
Or
What makes a balloon get inflated when air is filled in it?

Answer: Air exerts pressure on the inner walls of balloon causes it to expand and get inflated.

Question 4: Explain why, a wide steel belt is provided over the wheels of an army tank.

Answer: The wide belts increase the area and reduce the pressure. This avoids sinking of the tank into the ground.

Question 5: Which force makes a rolling ball stop on its own?
Or
Why is a ball rolling along the ground gradually slows down and finally comes to rest?
Or
What makes a ball to stop when it is rolling?
Or
Why does a moving ball stop?

Answer: It is the force of friction between the surface of the ball and the ground that brings the moving ball to rest.

Question 6: What is muscular force? Give one example.

Answer: The force resulting due to the action of muscles is known as the muscular force. Example: lifting a bucket of water.

Question 7: What is described as a change in its state of motion?

Answer: A change in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion.

Question 8: What is meant by a contact force? Give an example.

Answer: Force that can be applied only when it is in contact with an object, it is also called a contact force. Example: lifting a bucket of water.

Question 9: Explain, why magnetic force is said to be a non-contact force.

Answer: A magnet can exert a force on another magnet without being in contact with it. The force exerted by a magnet is an example of a non-contact force.

Question 10: Why do the shape and size of balloon change when filled with air or water?

Answer: The high pressure produced by the gas or water molecules on the walls of balloon causes it to expand and get inflated.

Question 11: Why water comes out more slowly from an upstairs tap than from a similar tap downstairs?

Answer: This is so because water pressure is less in the upstairs tap due to lesser depth.

Question 12: Why are we not crushed by atmospheric pressure?
Or
If the pressure due to the atmosphere is so great, then why are we not crushed by it?

Answer: We do not get crushed because the atmospheric pressure acting on our body from outside is balanced by the blood pressure acting from inside.

Question 13: What is frictional force?
Or
What is friction?

Answer: The force that always acts on all the moving object and its direction is always opposite to the direction of motion is called frictional force (or friction).

Question 14: Why does a sharp knife cut better than a blunt knife?

Ans. Lesser the area, larger the pressure for the same force applied. Therefore, it is easier to cut with sharp knives than blunt ones, with same force.

Question 15: What is meant by atmospheric pressure?

Answer: The atmospheric air extends up to many kilometres above the surface of the earth. The pressure exerted by this air is known as atmospheric pressure.

Question 16: Give two examples of contact forces.

Answer: The examples of contact forces are:

Muscular force
Frictional force

Question 17: 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, he uses his muscular force. This muscular force changes the shape of the iron.

Question 18: What is meant by a non-contact force? Give an example.

Answer: A force that can be exerted by an object even from a distance is called a non-contact force. The force exerted by a magnet is an example of a non-contact force.

Question 19: What do you understand by state of Motion?

Ans. The state of motion of an object is described by its speed and the direction of motion. The state of rest is considered to be the state of zero speed. An object may be at rest or in motion; both are its states of motion.

Question 20: Why the tools meant for cutting and piercing always have sharp edges?

Answer: The cutting edges of tools like blades, knives etc., are provided with sharp edges to cut the things easily because the smaller the area, larger the pressure on a surface for the same force.

Question 22: Explain why, the tip of a sewing needle is sharp.

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

Question 23: “When a ball is dropped from a height, its speed increases gradually.” Name the force which causes this change in speed.

Answer: The force of gravity causes all the objects to fall towards the earth. Thus, when a ball is dropped from a height, its speed increases gradually because of the force of gravity acting on it.

Question 24: Explain why, porters place a thick, round piece of cloth on their heads when they have to carry heavy loads.

Answer: By this they increase the area of contact of the load with their head. So, the pressure on their head is reduced and they find it easier to carry the load.

Question 25: Explain now why shoulder bags are provided with broad straps and not thin straps?
Or
Why the straps of school bags are made broader?
Or
Why is it comfortable to lift a school bag with broad straps than thin straps?

Answer: Shoulder bags are provided with broad straps and not thin strap so that the weight of the bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder.

Question 26: A small device pulls iron nails from a distance. Which type of force is involved in this process?

Answer: Magnetic force is involved in this process. The magnetic force acts even from a distance. The magnet exerts a magnetic force on objects made of iron, steel, nickel and cobalt.

Question 27: Write one example where force changes the direction of a moving object.

Answer: In cricket, a batsman plays his or her shot by applying a force on the ball with the bat. In this example the speed and the direction of the moving ball change due to the application of a force.

Question 28: 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?

Answer: If there is no movement on either side, the force applied from both side is equal in magnitude.

Question 29: Give three examples of non-contact forces.

Answer: The examples of non-contact forces are:

Magnetic force
Electrostatic force
Gravitational force

Long Answer Type Questions

Question 1: Explain why, frictional force is said to be a contact force.

Answer: The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion. Since the force of friction arises due to contact between surfaces, it is an example of a contact force.

Question 2: Explain why the walls of a dam are thicker near the bottom than at the top.

Ans. The walls of a dam are much thicker at the bottom than it is at the top. This is because the pressure of the water is much greater deeper down and the dam needs to be thick at the bottom so that it is strong enough to withstand this larger pressure.

Question 3: 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 acting on the rocket are the force of gravity, which pulls the rocket towards the ground, and the force of friction due to earth’s atmosphere, which opposes its motion.

Question 4: What is meant by gravitational force (or force of gravity)? Give its one example.

Answer: Every object in the universe, whether small or large, exerts a force on every other object. This force is known as the gravitational force. Example: Water begins to flow towards the ground as soon as we open a tap.

Question 5: Give two examples of situations in which applied force causes a change in the shape of an object.

Answer: Two examples in which applied force causes a change in the shape of an object are:

The shape of dough changes on pressing with rolling pin to make chapatis.
The shape of a toothpaste tube changes when we squeeze it.

Question 6: What are the various effects of force?

Answer: Effects of force are:

It may make an object move from rest.
It may change the speed of an object if it is moving.
It may change the direction of motion of an object.
It may bring about a change in the shape of an object.

Question 7: Why a fountain of water is created at the leaking joints or holes of pipes of the main water supply line?

Answer: Many times we see a fountain of water rushing out of the leaking joints (or holes) in the pipes of main water supply line. It is due to the very high pressure exerted by water on the sides (or walls) of the pipes that such a fountain of water is formed.

Question 8: Why does the liquid rise in a syringe when its piston is pulled up?
Or
How does a liquid medicine enter a syringe?

Answer: When the nozzle of a syringe is dipped in a liquid and its piston is withdrawn, the pressure inside the syringe is lowered. The greater atmospheric pressure acting on the surface of the liquid pushes the liquid up into the syringe.

Question 9: Why nose bleeding occurs at high altitudes?
Or
Why do mountaineers suffer from nose bleeding at high altitudes?

Answer: At high altitudes, the atmospheric pressure becomes much less than our blood pressure. Since, our blood is at higher pressure than outside pressure, therefore, some of the blood vessels in our body burst and nose bleeding takes place at high altitudes.

Question 10: Does application of a force would always result in a change in the state of motion of the object?

Answer: No. Many a time application of force does not result in a change in the state of motion. For example, a heavy box may not move at all even if you apply the maximum force that you can exert. Again, no effect of force is observed when you try to push a wall.

Question 11: 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: In this case, muscular force is acting in upward direction and the force of gravity is acting in downward direction. The two forces are equal in magnitude but opposite in direction. Therefore, the net force on the bucket is zero. Hence, there is no change in its state of motion.

Question 12: Why atmospheric pressure decreases as we go higher up above the earth’s surface?

Answer: The pressure at any level in the atmosphere may be interpreted as the total weight of the air above a unit area at any elevation. Since most of the atmosphere’s molecules are held close to the earth’s surface by the force of gravity, air pressure decreases as we go higher up above the earth’s surface.

Question 13: What happens when a charged plastic straw is brought near another uncharged plastic straw?

Answer: A straw is said to have acquired electrostatic charge after it has been rubbed with a sheet of paper. Such a straw is an example of a charged body. A straw rubbed with paper attracts another straw but repels it if it has also been rubbed with a sheet of paper.

Question 14: Why does a plastic comb rubbed with dry hair attract tiny pieces of paper?

Answer: When a plastic comb is rubbed in dry hair, the comb gets electric charges by friction. And when this electrically charged comb is brought near tiny pieces of paper, it exerts an electrostatic force of attraction on them. Due to this electrostatic force, the tiny pieces of paper move towards the comb and stick to it.

Question 15: What is meant by saying that force is due to interaction?

Answer: Example: Suppose a man is standing behind a stationary car. The car will not move just due to his presence. Suppose the man now begins to push the car, that is, he applies a force on it. The car may begin to move in the direction of the applied force.

From the above example, we can infer that at least two objects must interact for a force to come into play.

Question 16: If the area of my head were 10 cm × 10 cm, how much weight of air would I be carrying on my head? Why don’t we feel the weight of such a huge amount of air on us?

Answer: The weight of air in a column of the height of the atmosphere and area 10 cm × 10 cm is as large as 1000 kg. The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric pressure and cancels the pressure from outside.

Question 17: If a vacuum is created between two Magdeburg hemispheres joined together, they cannot be separated easily. Give reason.
Or
Why do Magdeburg hemispheres stick together?

Answer: This is due to the fact that since there is no air inside, the unopposed atmospheric pressure acting over the whole surface of the hemispheres from outside presses them very, very hard and does not allow them to be separated.

Question 18: How does a rubber sucker work?

Answer: 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 atmosphere acts on it. To pull the sucker off the surface, the applied force should be large enough to overcome the atmospheric pressure. In fact, it would not be possible for any human being to pull the sucker off the surface if there were no air at all between the sucker and the surface.

Question 19: Give two examples each of situations in which you push or pull to change the state of motion of objects.

Answer: Objects are pushed or pull in order to change the state of motion.

Examples of push are:

We push the door to open it.
A football player kicks the ball.

Examples of pull are:

We pull the door to close it.
Opening a drawer.

Question 20: 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 inflated balloon is rubbed with a piece of synthetic cloth, it gets electrically charged. A charged body exerts an electrostatic force on uncharged body. Thus, when the charged balloon is pressed against a wall, it sticks to the wall because of the electrostatic force acting between the charged balloon and the wall.

Question 21: Explain why, when a person stands on a cushion, the depression is much more than when he lies down on it.

Answer: When a man stands on a cushion then only his two feet are in contact with the cushion. Due to this, the weight of man falls on a small area of cushion producing a large pressure. This large pressure causes a big depression in the cushion. On the other hand, when the same man is lying on the cushion, then his whole body is in contact with the cushion. In this case, the weight of man falls on a much larger area of the cushion producing much smaller pressure. And this smaller pressure produces a very little depression in the cushion.

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