Force and Pressure - Exemplar Solutions 1

CBSE Class 8 Science
NCERT Exemplar Solutions
CHAPTER – 11
Force and Pressure

MULTIPLE CHOICE QUESTIONS

1. In Fig 11.1, two boys, A and B are shown applying force on a block. If the block moves towards the right, which one of the following statements is correct?

(a) Magnitude of force applied by A is greater than that of B.

(b) Magnitude of force applied by A is smaller than that of B.

(d) Magnitude of force applied by A is equal to that of B.

Ans: (a) Magnitude of force applied by A is greater than that of B.

Explanation: When the respective forces are applied on the block, it will move towards the direction in which a greater force is being applied. The net force will move the block. Since the block moves towards the right, the magnitude of force being applied by A is greater than that of B.

2. In the circuit shown in Fig.11.2, when the circuit is completed, the hammer strikes the gong. Which of the following force is responsible for the movement of hammer?

(a) gravitational force alone

(b) electrostatic force alone

(d) frictional force alone

Ans: (c) magnetic force alone

Explanation: When the circuit is completed, an electric current flows in the circuit, The electric current produces a magnetic force. The magnetic force is responsible for the movement of the hammer. When the magnetic force is produced, the hammer strikes the gong.

3. During dry weather, while combing hair, sometimes we experience hair flying apart. The force responsible for this is:

(a) force of gravity.

(b) electrostatic force.

(d) magnetic force.

Ans: (b) electrostatic force.

Explanation: This phenomenon occurs due to electrostatic force of charges. When two bodies are rubbed together, there is friction between them. The bodies get charged by friction.

While combing hair during dry weather, charge is generated on the comb and the hair. It is because of the electrostatic forces between the hair that we experience hair flying apart.

4. Fig. 11.3 shows a container filled with water. Which of the following statements is correct about pressure of water?

(a) Pressure at A > Pressure at B > Pressure at C

(b) Pressure at A = Pressure at B = Pressure at C

(d) Pressure at A < Pressure at B < Pressure at C

Ans: (d) Pressure at A < Pressure at B < Pressure at C.

Explanation: Pressure inside a liquid increases with the depth. Point C is at a greater depth as compared to point A and point B.Therefore, pressure at point C is the greatest and pressure at point A is the least.

5. Two objects repel each other. This repulsion could be due to:

(a) frictional force only

(b) electrostatic force only

(d) either a magnetic or an electrostatic force

Ans: (d) either a magnetic or an electrostatic force

Explanation: The repulsion between two objects can be due to either a magnetic force or an electrostatic force. Both are non-contact forces. In magnetism, like poles repel each other whereas unlike poles attract each other. Electrostatic force, like magnetic force, also acts from a distance. Like charges repel each other whereas unlike charges attract each other.

6. Which one of the following forces is a contact force?

(a) force of gravity

(b) force of friction

(d) electrostatic force

Ans: (b) force of friction

Explanation: Force of friction is a contact force. It is generated between the surfaces of two objects due to obstruction by one of the objects to the other object.

7. A water tank has four taps fixed at points A, B, C, D as shown in Fig. 11.4. The water will flow out at the same pressure from taps at:

(a) B and C

(b) A and B

(d) A and C

Ans: (a) B and C

Explanation: The water will flow out at the same pressure from taps at points B and C because B and C are at same height. At the same depth, the pressure exerted by a liquid is equal in all directions.

8. A brick is kept in three different ways on a table as shown in Fig. 11.5. The pressure exerted by the brick on the table will be:

(a) maximum in position A

(b) maximum in position B

(d) equal in all cases.

Ans: (a) maximum in position A

Explanation: The effect of force depends on the area of contact. For a given amount of force, lesser area will result into a greater pressure. Therefore, the pressure exerted by the brick on the table will be maximum in position A and minimum in position C.