Chapter 11: Force and Pressure

Science

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   Chapter 11: Force and Pressure

  Class: VIII

   Exemplar Solutions

 

MULTIPLE CHOICE QUESTIONS

Question 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.

 (c) Net force on the block is towards A.

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

Answer 1 (a)

Question 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   (c) magnetic force alone

(b) electrostatic force alone  (d) frictional force alone

Answer 2 (c)

Question  3

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

(a) force of gravity.  (c) force of friction.

(b) electrostatic force.   (d) magnetic force.

Answer 3 (b)

Question  4

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

• Pressure at A> Pressure at B> Pressure at C
• Pressure at A=Pressure at B=Pressure at C
• Pressure at A< Pressure at B>Pressure at C
• Pressure at A< Pressure at B

Answer 4 (d)

Question 5

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

 (a) frictional force only

(b) electrostatic force only

(c) magnetic force only

 (d) either a magnetic or an electrostatic force

Answer 5 (d)

Question  6

 Which one of the following forces is a contact force?

(a) force of gravity   (c) magnetic force

(b) force of friction  (d) electrostatic force

Answer 6 (b)

Question 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  (c) C and D

(b) A and B  (d) A and C

Answer 7 (a)

Question 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  (c) maximum in position C

(b) maximum in position B  (d) equal in all cases.

Answer 8 (a)

 

VERY SHORT ANSWER QUESTIONS

Question 9

A ball of dough is rolled into a flat chapatti. Name the force exerted to change the shape of the dough.

Answer 9

Muscular force.

Question 10

Where do we apply a force while walking?

Answer 10

While walking we apply force on the ground.

Question 11

 A girl is pushing a box towards east direction. In which direction should her friend push the box so that it moves faster in the same direction?

Answer 11

Towards east.

Question 12

 

In the circuit shown in Fig.11.6, when the key is closed, the compass needle placed in the match box deflects. Name the force which causes this deflection.

Answer 12

Magnetic force.

Question 13

 During dry weather, clothes made of synthetic fibre often stick to the skin. Which type of force is responsible for this phenomenon?

Answer 13

Electrostatic force.

 

Question 14

 While sieving grains, small pieces fall down. Which force pulls them down?

Answer 14

Force of gravity

Question  15

 Does force of gravity act on dust particles?

Answer 15

Yes

Question 16

 A gas filled balloon moves up. Is the upward force acting on it larger or smaller than the force of gravity?

Answer 16

Upward force is larger than the force of gravity.

Question 17

 Does the force of gravitation exist between two astronauts in space?

Answer 17

Yes

SHORT ANSWER QUESTIONS

Question 18

 A chapati maker is a machine which converts balls of dough into chapati’s. What effect of force comes into play in this process?

Answer 18

 The force changes the shape of the dough.

Question 19

 

 Fig.11.7 shows a man with a parachute. Name the force which is responsible for his downward motion. Will he come down with the same speed without the parachute?

Answer 19

Force of gravity. No, without the parachute his speed will be higher.

Question 20

Two persons are applying forces on two opposite sides of a moving cart. The cart still moves with the same speed in the same direction. What do you infer about the magnitudes and direction of the forces applied.

Answer 20

Both the forces are of equal magnitudes and applied in the opposite directions.

Question 21

Two thermocol balls held close to each other move away from each other. When they are released, name the force which might be responsible for this phenomenon. Explain.

Answer 21

Electrostatic force. The balls have similar charges. They move away due to repulsion between similar charges.

Question 22

 Fruits detached from a tree fall down due to force of gravity. We know that a force arises due to interaction between two objects. Name the objects interacting in this case.

Answer 22

Earth and fruits.

Question 23

 A man is pushing a cart down a slope. Suddenly the cart starts moving faster and he wants to slow it down. What should he do?

Answer 23

He should apply a force to pull the cart up the slope.

Question 24

Fig. 11.8 shows a car sticking to an electromagnet. Name the forces acting on the car? Which one of them is larger?

Answer 24

Magnetic force (in the upward direction) and force of gravity or the weight of the car (downward). Magnetic force is larger than the force of gravity.

 

LONG ANSWER QUESTIONS

Question 25

An archer shoots an arrow in the air horizontally. However, after moving some distance, the arrow falls to the ground. Name the initial force that sets the arrow in motion. Explain why the arrow ultimately falls down.

Answer 25

The archer stretches the string of the bow by applying muscular force. In the process the shape of the bow changes. When the string is released, it regains its original position that provides the initial force to set the arrow in motion. The force of gravity that acts on the arrow in the downward direction brings it to the ground.

Question 26

 It is difficult to cut cloth using a pair of scissors with blunt blades. Explain.

Answer 26

Blunt blades have larger area compared to the sharp-edged blades. Thus, the applied force produces a lower pressure in case of blunt blades, which makes it difficult to cut the cloth.

Question 27

 Two rods of the same weight and equal length have different thickness. They are held vertically on the surface of sand as shown in Fig.11.9. Which one of them will sink move? Why?

Answer 27

Rod B will go deeper as it has a smaller area of contact, therefore the same force (weight of the rod) produces more pressure. In case of rod A the same force produces less pressure.

Question 28

Two women are of the same weight. One wears sandals with pointed heels while the other wears sandals with flat soles. Which one would feel more comfortable while walking on a sandy beach? Give reasons for your answer.

Answer 28

The woman wearing sandals with flat soles will feel more comfortable while walking on the sandy beach. The flat soles have larger area compared to the sandals with pointed heels. Since the two women are of the same weight, they will apply same force on the ground. Therefore, the pressure exerted by the pointed heels will be more compared to that with sandals having flat soles. As a result the pointed heel sandals will sink more in the sand than the flat sole sandals. Hence, walking with flat sole sandals will be more comfortable.

Question 29

 It is much easier to burst an inflated balloon with a needle than by a finger. Explain.

Answer 29

When we prick the surface of an inflated balloon with a needle it exerts a larger pressure because it has a smaller area of contact compared to the finger. The large pressure pierces the surface of the balloon easily.

Question 30

Observe the vessels A, B, C and D shown in Fig.11.10 carefully.

Volume of water taken in each vessel is as shown. Arrange them in the order of decreasing pressure at the base of each vessel. Explain.

Answer 30

B, D, A, C. Because pressure of a liquid column depends upon the height of the liquid column and not on volume of the liquid.