Lesson: Force

Exemplar:

Question: 1

In the figure below, 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.

Solution:

Question: 2

In the circuit shown in figure below, 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

Solution:

Question: 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

Solution:

Question: 4

Figure below 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

Solution:

Question: 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

Solution:

Question: 6

Which one of the following forces is a contact force?

(a) Force of gravity

(b) Force of friction

(d) Electrostatic force

Solution:

Question: 7

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

(a) B and C

(b) A and B

(d) A and C

Solution:

Question: 8

A brick is kept in three different ways on a table as shown in the figures below.

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

Solution:

VERY SHORT ANSWER QUESTIONS (9)

Question: 1

A ball of dough is rolled into a flat chapatti.

Name the force exerted to change the shape of the dough.

Solution:

Muscular force is exerted to change the shape of the dough.

Question: 2

Where do we apply a force while walking?

Solution:

We apply force on the ground while walking.

Question: 3

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?

Solution:

Her friend should push the box towards east direction.

Question: 4

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.

Solution:

Magnetic force causes the deflection.

Question: 5

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

Solution:

Due to the electrostatic force, clothes made of synthetic fibre often stick to the skin.

Question: 6

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

Solution:

Gravitational force

Question: 7

Does force of gravity act on dust particles?

Solution:

Yes

Question: 8

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

Solution:

The upward force acting on the balloon is larger than the force of gravity.

Question: 9

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

Solution:

Yes

SHORT ANSWER QUESTIONS (8)

Question: 1

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

Solution:

The force changes the shape of the dough.

Question: 2

The figure below 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?

Solution:

Gravitational force is responsible for his downward motion. No, without the parachute his speed will be greater.

Question: 3

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?

Solution:

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

Question: 4

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.

Solution:

The force responsible for the phenomenon is electrostatic force. Both the balls have similar charges and they move away due to repulsion between similar charges.

Question: 5

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.

Solution:

In this case, the objects interacting are the earth and fruits.

Question: 6

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?

Solution:

He can apply a force to pull the cart up the slope in order to balance the force due to gravity.

Question: 7

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

Solution:

Forces acting on the car:

(a) Magnetic force (in the upward direction) and

(b) Force of gravity or the weight of the car (downward).

Magnetic force is larger than the force of gravity.

LONG ANSWER QUESTIONS (6)

Question: 1

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.

Solution:

It is the muscular force which set the arrow in motion. After that, the frictional force of air and gravitational force acts on it. Due to these forces, the speed decreases and the arrow ultimately falls to the ground.

Question: 2

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

Solution:

The blunt blades of a pair of scissors have a larger area than one with sharp-edged blades. Due to larger area, the applied force produces a lower pressure. This makes it difficult to cut the cloth with a pair of scissors having blunt blades.

Question: 3

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

Solution:

Rod B will go deeper because 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 lesser pressure.

Question: 4

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.

Solution:

As we know that pressure exerted by the body is inversely proportional to the area where force is applied, hence the woman wearing sandals with flat soles will feel more comfortable. The flat soles have larger area compared to the sandals with pointed heels. Therefore, the pressure exerted by the pointed heels will be more compared to the sandals having flat soles. As a result, the sandals with pointed heels will sink more in the sand than the sandals with flat soles.

Question: 5

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

Solution:

Pressure exerted by a body is inversely proportional to the area where force is applied. Therefore, a needle exerts larger pressure because it has smaller area of contact and a finger exerts lesser pressure due to its larger area of contact. Hence, it becomes much easier to burst an inflated balloon with a needle than by a finger.

Question: 6

Observe the vessels A, B, C and D shown in figure 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.

Solution:

D, B, A, C. This is because pressure of the liquid column depends upon the height of the liquid column and not on the volume of the liquid column.