RBSE Class 8 Science Important Questions Chapter 11 Force and Pressure

Rajasthan Board RBSE Class 8 Science Important Questions Chapter 11 Force and Pressure Questions and Answers.

RBSE Class 8 Science Chapter 11 Important Questions Force and Pressure

Objective Questions

Question 1.    
Which is / are the actions of a force
(a) A body at rest can be brought into motion by applying a force.
(b) Force can change the speed of a moving body.
(c) Force can change the direction of motion of a moving body.
(d) Force is capable of doing all the actions given in (a), (b) and (c).
Answer:
(d) Force is capable of doing all the actions given in (a), (b) and (c).

Question 2.    
Muscular force is
(a) Contact force 
(b) Non - contact force 
(c) Friction force 
(d) None of the above
Answer:
(a) Contact force 

Question 3.    
Which of the following is not the example of non - contact force
(a) Force due to a magnet on iron
(b) Electrostatic force
(c) Muscular force
(d) Gravitational force
Answer:
(c) Muscular force

Question 4.    
Formula for pressure is
(a) Pressure =\( \frac{\text { force }}{\text { area }}\)
(b) Pressure = \(\frac{\text { area }}{\text { force }}\)
(c) Pressure = force x area
(d) (a) and (b) both
Answer:
(a) Pressure = \( \frac{\text { force }}{\text { area }}\)

Question 5.    
What is the weight of a body of man 225 kg
(a) 225 N    
(b) 2250 N
(c) 2500 N    
(d) Uncertain
Answer:
(b) 2250 N

Fill in the blanks 
1. A .......................... acts due to the interaction between the two bodies.
Answer:
force

2. Force acting per unit area is called ..........................
Answer:
pressure

3. All bodies attract each other with a force. This force is called .......................... force.
Answer:
gravitation.

True/False 

Read the following statements carefully and write 'T' for true and 'F' for false.
1. A force acts due to the interaction between the two / bodies.
Answer:
True

2. A force has only direction and not the magnitude.
Answer:
False

3. Force cannot change the shape of a body.
Answer:
False

4. Friction force only enables us to do all actions.
Answer:
False

5. Force acting due to muscles is known as muscular force.
Answer:
True

6. Force on a magnet due to other magnet is an example of non - contact force. 
Answer:
True

Match the words given in 'Column-A' with 'Column-B'
Question 1. 

Column-A	Column-B
(i) Gravitational force	(a) Force exerted by muscle
(ii) Electro – static force	(b) Pressure exerted by air
(iii) Atmospheric pressure	(c) Force exerted by charged objects
(iv) Muscular force	(d) The attraction force exerted by the earth

Answer:

Column-A	Column-B
(i) Gravitational force	(d) The attraction force exerted by the earth
(ii) Electro – static force	(c) Force exerted by charged objects
(iii) Atmospheric pressure	(b) Pressure exerted by air
(iv) Muscular force	(a) Force exerted by muscle

Very Short Answer Type Questions

Question 1. 
What is a force?
Answer:
The push or pull acting on a body is called a force.

Question 2. 
What is the tendency of a body in the absence of a force?
Answer:
In the absence of a force the body will remain at rest or in uniform motion in a straight line.

Question 3. 
A ball rolling on a floor comes to rest after some time. Which force is responsible for this?
Answer:
Frictional force is responsible for bringing the rolling ball to rest.

Question 4. 
If two opposite forces act on a body, what is the net force acting on the body? 
Answer:
Net force will be equal to the difference between the two forces.

Question 5. 
What do you mean by muscular force? 
Answer:
The force exerted by muscles of our body is called muscular force.

Question 6. 
What is electrostatic force?
Answer:
The force of attraction or repulsion between two charged bodies is called electrostatic force.

Question 7. 
Define a magnetic force.
Answer:
The force between two magnetic poles or a magnetic pole and a magnetic material (e.g. iron) is called a magnetic force.

Question 8. 
Name a force which acts between two bodies in contact.
Answer:
Frictional force.

Question 9. 
Give two examples in which the force is applied to change the motion of a body.
Answer:

• To hit or strike a body.
• To pull or push a body.

Question 10. 
What is pressure?
Answer:
Force acting per unit area on a body is called pressure.

Question 11. 
Give two examples of contact force. 
Answer:

• Muscular force 
• Frictional force. 

Question 12. 
What is a non - contact force?
Answer:
The force acting between two bodies, even when the bodies are not in contact is called non - contact force.

Question 13. 
Give examples of non - contact forces. 
Answer:

• Magnetic force
• Electrostatic force
• Gravitatiopal force.

Short Answer Type Questions

Question 1. 
What is a force? How does a force act due to interaction?
Answer:
Force: The push or pull acting on a body is called a force.

Force due to interaction:
We can say that interaction between at least two bodies is necessary for force to come into play. For example let us consider a person standing behind a car. This person cannot move the car unless he gives a physical push to the car. This action between the person and the car is called interaction and it results into the force between person and the car.

Question 2. 
What do you mean by the magnitude of a force? Explain.
Answer:
We knbw that when two persons push a body simultaneously in the same direction, the body moves easily and if both persons apply the forces in opposite direction it becomes difficult to move the body. On this basis we can infer that the forces applied in the same direction are added up and when forces are applied in the opposite direction, the net force is equal to the difference in the magnitude of forces. 
In this way we can say that a force can be greater or smaller than the other force. The strength of a force is measured by its magnitude. Therefore it is very important to have the knowledge of magnitude and direction, while determining a force.

Question 3. 
Explain by giving an example that a force (push or pull) can change the state of rest of a body. 
Answer:
A table is kept at rest in a room. It can be displaced by pushing or pulling. But if a similar push or pull is applied to a wall, then the position of wall cannot be changed. In this way we can say that a force can change the state of rest of a body or a force can make an effort to make a change.

Question 4. 
Explain by giving an example that a force can change the state of uniform motion of a body or a force can make an effort to change the state of uniform motion.
Answer:
A ball is moving with uniform velocity on a smooth surface. If a force is applied in opposite direction, the ball can be stopped. But this force cannot be used to stop a car. Hence we can say that a force can change the state of uniform motion of a body or can make an effort to change the state of uniform motion. 

Question 5. 
Explain by giving example that a force can change the shape of a body.
Answer:
Yes, it is true that a force can change the shape of a body. E.g. (i) If an inflated balloon (Ml of air) is pressed by hand, its shape changes, (ii) If a rubber ball is similarly pressed, its shape also changes, (iii) If a spring is pulled, its shape also changes.

Question 6. 
What do you mean by a contact force. Explain giving example.
Answer:
Contact force:
If a body is touched for applying a force, then the force is called contact force. E.g. If we want to lift a bucket full of water, then it is necessary to touch bucket for applying force. This force is produced due to the action of our muscles. This force is called muscular force. Hence, we can say that muscular force is a contact force. This force can be brought into action by contact only.

Question 7. 
What is a frictional force? How does it work? Explain.
Answer:
Frictional force:
The force between the sufaces of contact of two bodies is called frictional force. This force changes the state of motion of two bodies. The direction of this force is opposite to the direction of motion. It is also an example of contact force because it is produced between the surfaces of contact of two bodies.

Question 8. 
What is the kind of force between two magnets? Explain.
Answer:
We know that similar poles of two magnets repel each other and opposite poles of two magnets attract each other, therefore these forces are similar to push and pull forces. But these forces come into play without bringing the magnetic poles to come in contact. Hence this force is a non - contact force. Similarly the force between a magnet and iron is also a non - contact force.

Question 9. 
What is a gravitational force? Explain with example.
Answer:
All bodies (small or big) in the universe exert a force on each other. This force is known as gravitational force. It is an attractive force. This force acts on all bodies. It is acted on us also but we do not feel it. All bodies fall on earth because earth attracts them, E.g. falling of fruits and leaves from trees, falling of water when we open a tap, flow of water from high level to low level etc. are the examples of gravitational forces.

Question 10. 
Write the names of few fundamental forces of nature.
Answer:
The names of few fundamental forces are as follows:

1. Gravitational force acts between two masses.
2. Electrostatic force acts between two charged bodies.
3. Magnetic force acts between two magnetic poles or between a magnet and magnetic material (like iron).
4. Frictional force which acts between two surfaces of contact and it opposes the motion. 

Question 11. 
What are the main effects produced by forces?
Answer:
Following effects can be produced by forces:

1. A force can bring a body at rest into motion and a moving body into position of rest.
2. A force can change the speed of a body.
3. A force can change the direction of motion of a body.
4. A force can change the shape of a body.
5. A force can produce any of the above effects or all of the above effects.

Question 12. 
What is pressure? What is the importance of the area where the force is acting?
Answer:
Force acting per unit area of a surface is known as pressure.
\(\text { Pressure }=\frac{\text { Force }}{\text { Area on which it acts }}\)

It is clear from the formula that area appears in the denominator of the formula. Therefore for the same force, if area is small the pressure will be more. The area of the pointed side of a nail is very small as compared to its head. Therefore, when pointed side is put on a wooded board and the nail is hit on its head, large pressure is produced and the nail penetrates into the wood.

The porters while lifting heavy loads on their heads place a round piece of cloth on their heads. (Refer to the formula of pressure given above). The porter has increased the area (using a round piece of cloth) where the heavy load is to be placed so the pressure on his head is reduced and they find it easier to carry heavy loads.

Question 13. 
Why are the slings of school bags broad?
Answer:
School bags have wide straps made of canvas so that large area of the child's shoulder carry the load of the bag, causing less pressure on the shoulder and due to less pressure, the heavy bag is more easy to carry. On the other hand if the school bag has a thin strip of rope, then the weight of the school bag will fall on a small area on shoulder. This will generate more pressure on the child's shoulder and it will be very painful to carry heavy school bags.

Question 14. 
What do you mean by pressure due to a liquid?
Answer:
Molecules of a liquid have random motion within liquid. These molecules collide with each other and also with the walls of the container. The walls of the vessel and the bottom experience a force due to such collisions. Hence the force per unit is acting on the wall of the container and its bottom is known as pressure of the liquid.

Question 15. 
The total force acting on any person due to atmospheric pressure is 15 ton. How a person is able to bear such a high force, when we know that no body can carry even a ton of load.
Answer:
The skin of human body has a large number of tiny pores. Atmospheric air enters our body through these tiny pores. Therefore, air pressure due to the air present within human body balances the atmospheric pressure and we do not feel any pressure at all.

Question 16. 
When a person walks, the pressure exerted on earth is more than the pressure exerted when the person stands. Explain.
Answer:
When a person walks, at any time only one foot touches the ground. Due to this the force due to weight of whole body is borne by small area of only one foot. Therefore the pressure exerted is large. On the otherhand when the person stands the weight of whole body is borne by the area of both feet (about double the area). Hence the pressure exerted on earth is small.

Question 17. 
What do you understand by atmospheric pressure. Explain giving example.
Answer:
We know that all of us are surrounded by air. This envelop of air around us is known as atmosphere. The atmosphere extends upto several kilometers above the surface of earth. The pressure exerted by air present in atmosphere is called atmospheric pressure. We know that pressure is equal to the force per unit area. Let us imagine a unit area having a cylinder of height equal to the height of air atmosphere. Then the weight of air in this cylinder will be called atmospheric pressure. Atmospheric - pressure can be understood by the following activity.

Activity:
Take a good quality rubber sucker. Press it hard on the smooth plane (flat) surface so that all air goes out. By doing so the rubber sucker will stick to the surface because of the large atmospheric pressure acting on it. If we try to pull the sucker from the flat surface a large force will be needed. This force must be larger than the force due to atmospheric pressure.
This activity gives us a good idea regarding the atmospheric pressure.

Question 18. 
What will happen to the human body if the atmospheric pressure suddenly becomes zero?
Answer:
In the human body, equal and opposite pressure is applied by the blood to the atmospheric pressure, so that it equilibrate the outside pressure. If the atmospheric pressure suddenly falls to zero, then the blood will exert such a high pressure on the vessels that they will burst, resulting in the death of the person.

Essay Type Questions

Question 1. 
What is force? Give difference between contact and non - contact forces. 
Answer:
Force: A push or pull on any object is known as force.
Difference between contact and non - contact force

1. Contact force:
If in order to apply a force, contact is required, the force is called contact force. For example, when we lift a bucket of water, a contact is required between hand & bucket and the force is developed due to activity of our muscles. This force is called muscular force. Therefore muscular force is a contact force. This only operates through contact with object. Friction is also an example of contact force.

2. Non - contact force:
When a force is applied without a contact, it is called non - contact force. For example, two like poles of magnets repell each other while two unlike poles attract each other. This force operates without a contact, therefore it is a non - contact force. E.g. magnetic, electrostatic and gravitational force.

Question 2. 
What is electrostatic force? Explain an activity that demonstrate the electrostatic force with diagram.
Answer:
Electrostatic force:
The force exerted by a charged body or another charged or uncharged body in known as electrostatic force. This force comes into play even when the bodies are not in contact. 

Activity to demonstrate electrostatic force:
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. 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. We will see that the straw rubbed from a sheet of paper attracts the straw hanging from the table.

Now we will also rub the free end of the suspended piece of straw with a sheet of paper. Now, as before, bring both the straw closer. We will see that now the straw hanging from the table is repulsed. This, thus proved that a straw is said to have acquired electrostatic charge after it has been rubbed with a sheet of paper.

 
Question 3. 
Explain that liquids exert equal pressure on the walls of the container at the same depth. Draw essential diagram also.
Answer:
The fluid exert the same pressure on the vessel walls at the same depth. We can explain it by using the following experiment - Take an empty plastic bottle or a cylindrical container. You can take a used tin can or a used 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. Now fill the bottle with water. We will see that the water coming out of the four holes in the bottle falls at an equal distance from the bottle. This makes it clear that the fluid exerts the same pressure on the vessel walls at the same depth.