NCERT Exemplar for Class 9 Science - Force and Laws of Motion - Free PDF Download

Multiple Choice Questions

1. Which of the following statements is not correct for an object moving along a straight path in an accelerated motion?

a. Its speed keeps changing

b. Its velocity always changes

c. It always goes away from the earth

d. A force is always acting on it

Ans: The correct option is (c) It always goes away from the earth

Explanation: In order to move away from the earth, an object requires acceleration which should be more than the acceleration due to gravity of earth. Moving on a straight path is not sufficient for an object to escape the gravitation of the   earth.

2. According to the Newton's third law of motion, action and reaction

a. Always act on the same body

b. Always act on different bodies in opposite directions

c. Have the same magnitude and directions

d. Act on either body at normal to each other

Ans: The correct option is (b) always act on different bodies in opposite directions

Explanation:  Both action and reaction act on different bodies but in opposite directions. 

3. A goalkeeper in a game of football pulls his hands backward after holding the ball shot at the goal. This enables the goalkeeper to

a. Exert a larger force on the ball

b. Reduce the force exerted by the ball on hands

c. Increase the rate of change of momentum

d. Decrease the rate of change of momentum

Ans: The correct option is (b) reduce the force exerted by the ball on hands

Explanation: Pulling hand backward gives enough time to reduce the momentum of the ball which thus helps in reducing the force exerted by the ball on hands.

4. The inertia of an object tends to cause the object

a. To increase its speed

b. To decrease its speed

c. To resist any change in its state of motion

d. To decelerate due to friction

Ans: The correct option is (c), to resist any change in its state of motion.

Explanation: Inertia is understood as the property because of which an object resists any change in its state of motion.

5. A passenger in a moving train tosses a coin which falls behind him. It means that motion of the train is

a. Accelerated

b. Uniform

c. Retarded

d. Along circular tracks

Ans: The correct option is (a) accelerated

Explanation: If the motion of the train was uniform, the coin would have fallen in the passenger’s hand. If the motion had been retarded, the coin would have fallen ahead of the passenger. So, the motion is accelerated because it falls behind him.

6. An object of mass $2 \mathrm{~kg}$ is sliding with a constant velocity of $4 \mathrm{~ms}^{-1}$ on a frictionless horizontal table. The force required to keep the object moving with the same velocity is

(a) $32 \mathrm{~N}$

(b) $0 \mathrm{~N}$

(c) $2 \mathrm{~N}$ (d) $8 \mathrm{~N}$

Ans: The correct option is (b) $0 \mathrm{~N}$

Explanation: When there is no friction opposing the motion, no force is required to keep the object in a uniform motion.

$x \leq 8.5$

7. Rocket works on the principle of conservation of

a. Mass

b. Energy

c. Momentum

d. Velocity

Ans: The correct option is (c) momentum

Explanation- High velocity of hot gasses coming out from the rocket is balanced by the upward movement of the rocket. This gas has large momentum and an equal momentum is acted to the rocket in opposite direction due to which the rocket moves upward. Their conservation of momentum is working.

8. A water tanker filled up to 2/3 of its height is moving with a uniform speed. On sudden application of the brake, the water in the tank would

a. Move backward

b. Move forward

c. Be unaffected

d. Rise upwards

Ans: The correct option is (b) move forward.

Explanation: By sudden application of brake, the tanker would come to rest however, water would remain in motion hence it would move forward.

Short Answer Questions

9. There are three solid balls made up of aluminium, steel and wood, of the same shape and same volume. Which of them would have the highest inertia?

Ans: The correct answer is steel.

Explanation: Mass is a measurement of inertia then the ball of same shape and size having more mass than other balls will have highest inertia. As steel has the highest density and large mass, therefore, it has the highest inertia.

10. Two balls of the same size but of different materials, rubber and iron are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for your answer.

Ans: Yes, the balls would start rolling in the direction in which the train was moving. With the application of brakes, the train would come to rest but due to inertia the balls try to remain in motion, therefore, they begin to roll. Since the masses of the balls are not the same, therefore, the inertia of the iron ball is greater than the rubber ball due to mass. Thus, the rubber ball will move fast.

11. Two identical bullets are fired one by a light rifle and another by a heavy rifle with the same force. Which rifle will hurt the shoulder more and why?

Ans: According to the law of conservation of momentum; the momentum of the bullet forward will be equal to the momentum of the rifle backward. In the case of a light rifle; velocity will be more than the velocity of a heavier rifle because of mass so that momentum (product of mass and velocity) for both shall not be equal. Due to this, the lighter rifle will hurt the shoulder more. In both cases it has been called recoiled.

12. A horse continues to apply a force in order to move a cart with a constant speed. Explain why?
Ans: When a cart is moving on the road, it needs to encounter friction. In order to maintain a constant speed, some force needs to be applied continuously to overcome this friction. Therefore, the horse needs to apply a force continuously to move the cart with a constant speed.

13. Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed v again before striking the ground. It implies that the magnitude of the initial and final momentums of the ball are the same. Yet, it is not an example of conservation of momentum. Explain why?

Ans: The law of conservation of momentum is applicable to isolated systems. In this case, the change in velocity is due to the gravitational force of earth.

14. Velocity versus time graph of a ball of mass 50 g rolling on a concrete floor is shown in Fig. 9.1. Calculate the acceleration and frictional force of the floor on the ball.

Ans: Acceleration

$\text { Force }=m a=50 \text { multiply by } 10$ and dividing it by 100

15. A truck of mass M is moved under a force F. If the truck is then loaded with an object equal to the mass of the truck and the driving force is halved, then how does the acceleration change?

Ans: Acceleration can be given as follow:

$\mathrm{F=ma}$

When mass is doubled and force is halved; $(F=F / 2$ AND $M=2 M$ Since mass of truck and mass of object both. In other words mass is inversely proportional to acceleration so if mas is 4 then acceleration will be $1 / 4)$

Or So, acceleration becomes one-fourth.

16. Two friends on roller-skates are standing $5 \mathrm{~m}$ apart facing each other. One of them throws a ball of $2 \mathrm{~kg}$ towards the other, who catches it, how will this activity affect the position of the two? Explain your answer.

Ans: The separation between them will increase. Initially, momentum of both of them is nil as they are at rest. To conserve momentum, one of them who throws the ball would move backward and the other one would experience a net force after catching it so he would move backwards (in the direction of the force).

17. Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. Explain the principle on which it works.

Ans: The working of the rotation of sprinkler is based on Newton’s third law of motion. When water comes out of the nozzle of the sprinkler, an equal and opposite reaction force comes into play hence, the sprinkler starts rotating.

Long Answer Questions

18. Using Newton's second law of motion, derive the relation between force and acceleration. A bullet of $10 \mathrm{~g}$ strikes a sandbag at a speed of $10^{3} \mathrm{~ms}^{-1}$ and gets embedded after travelling $5 \mathrm{~cm}$. Calculate

1.  The resistive force exerted by the sand on the bullet

2.  The time taken by the bullet to come to rest.

Ans: The second law of motion states that the rate of change of momentum of an object is proportional to the applied force

Derivation:

Let initial and final momentum of the object are $p_{1}=m u$ and $p_{2=}=m v$ respectively.

The change in momentum $p=m v-m u$

$p=m \times(v-u)$

The rate of change of momentum is directly proportional to applied force $F$

i.e.

$F=k m \cdot a$

The unit of force is so chosen that the value of the constant, $k$ becomes one.

For this, one unit of force is defined as the amount that produces an acceleration of $1 \mathrm{~ms}^{-2}$ in an object of $1 \mathrm{~kg}$ mass. That is,

1 unit of force $=k x$

$(1 \mathrm{~kg}) \times(1 \mathrm{~m} \mathrm{~s})$

Thus, the value of $k$ becomes $1 .$

19. Derive the unit of force using Newton's second law of motion. A force of $5 \mathrm{~N}$ produces an acceleration of $8 \mathrm{~ms}^{-2}$ on a mass $\mathrm{m}_{1}$ and an acceleration of $24 \mathrm{~ms}^{-2}$ on a mass $m_{2}$. What acceleration would the same force provide if both the masses are tied together?

Ans: According to Newton's second law:

$F=m a=(k g)\left(m s^{-2}\right)=k g m s^{-2}$

It Is also called practical unit of force

This absolute unit of force is called newton. Its symbol is $N$.

If masses are tied together, then

$\mathrm{M}=\mathrm{m}_{1}+\mathrm{m}_{2}$

Acceleration produced in $\mathrm{M}$,

$a=F M$

20. What is momentum? Write its SI unit. Interpret force in terms of momentum. Represent the following graphically.

a. Momentum versus velocity when mass is fixed.

b. Momentum versus mass when velocity is constant

Ans:  Momentum is the product of mass of an object to its velocity.

Momentum $(P)=\operatorname{mass}(m) \times \operatorname{velocity}(v)$

SI unit of momentum is $\mathrm{kg} \mathrm{ms}^{-1}$

We can interpret force by Newton's second law which states that force is equal to rate of change of linear momentum.

Force $=$ Rate of change in linear momentum

Graph of momentum versus mass:

NCERT Exemplar Class 9 - Chapter 9 - Force and Laws of Motion

Chapter 9 of Class 9th science NCERT, that is, Force and Laws of Motion in continuation with the 8th chapter that was Motion. Chapter 9 basically deals with the causes of motion. The chapter is very important from not only examination point of view but also keeping higher competitive exams in mind. Certain essential concepts that are discussed in Chapter 9 science NCERT are:

• The tendency of an object to resist change in its state of rest or motion is referred to as inertia.

• The mass of an object is the measure of its inertia and have SI unit as kilogram (kg)

• The motion of objects is always opposed by the force of friction.

• The SI unit of force is Newton (N) or kg m/s2

• One Newton is said to produce an acceleration of 1 m/s2 on an object of mass 1 kg.

• The momentum of an object is defined as the product of mass of the object and its velocity.

• The momentum of an object has the same direction as the velocity of the object.

• Total momentum remains conserved when considering an isolated system, that is, where there is no external force applied.

Laws of Motion described in Chapter 9 of Class 9th Science NCERT

Chapter 9 of class 9th NCERT deals with the three major laws of motion. It is expected for each student to know these laws of motion on his/her fingertips. The three laws of motion described in NCERT are:

First law of Motion: The first law of motion states that an object continues to be in its state of rest or motion along a straight line until and unless some unbalanced force is applied to it.

Second Law of Motion: The second law of motion states that the rate of change of momentum of an object is proportional to the force applied to it and it is in the direction of the force.

Third Law of Motion: The third law of motion states that to every action of one body, there is an equal and opposite reaction obtained from the other body.

Why should I download the Science NCERT Exemplar for Chapter 9?

It is very advantageous to download NCERT exemplar for Class 9th. Here are certain reasons on why you should definitely download the NCERT Exemplar:

• The NCERT exemplar is available for free, therefore, it is no harm to download and study the exemplar.

• It is accessible 24 X 7 and all 365 days of the year.

• It carries solutions to not only Chapter 9 of Science but for all the chapters present in the Science NCERT textbook.

• The exemplar comes with proper explanations to all the questions present in the booklet.