NCERT Exemplar Solution Class 9 Science Chapter 9
NCERT Exemplar Solution Class 9 Science Chapter 9 Force and Laws of Motion all Questions Answer Solution. NCERT Exemplar Class 9 Science Chapter 9 Solution PDF.
NCERT Exemplar Solution Class 9 Science Chapter 9: Overview
NCERT Exemplar Solution Class 9 Science Chapter 9 |
|
Board |
NCERT |
Topic |
Exemplar Problem Solution |
Class |
9 |
Subject |
Science |
Chapter |
9 |
Chapter Name |
Force and Laws of Motion |
NCERT Exemplar Solution Class 9 Science Chapter 9 Force and Laws of Motion
Multiple Choice Question
1) Which of the following statement is not correct for an object moving along a straight path in an accelerated motion?
a) It’s speed keeps changing
b) It’s velocity always changes
c) It always goes away from the earth
d) A force is always acting on it.
Answer:- c) it always goes away from the earth.
C) is the wrong statement because when an object goes away from the earth it should be always accelerated which is more than the acceleration due to gravity. For moving away from the gravity of earth and escape from it only moving in straight line is not enough.
2) According to the 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 some magnitude and directions
d) Act on either body at normal to each other.
Answer:- b) always act on the different bodies in opposite directions.
According to the Newton’s third law of motion every action has an equal and opposite reaction.
3) A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goal keeper to
a) Exert 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
Answer:- b) Reduce the force exerted by the balls on hands.
When the goal keeper pull The hands backward it minimizes the momentum of the ball and thus it reduces the force exerted by the ball over the hands
4) The inertia of an object tends to cause the object
a) To increase its speed
b) To decrease it’s speed
c) To resist any change in its state of motion
d) To decelerate due to friction
Answer:- c) to resist any change in its state of motion.
Inertia of an object will resist the change in its state of motion. Change in any state of an object is dependent over the external force.
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) Retarted
d) Along circular tracks
Answer:- a) accelerated
A coin in the moving train if felled behind the person it means the train is accelerated. If the train would be in the uniform motion then the coin would fell on the same point and if it would be retarted then the coin would fallen in front of the person. Along the circular track coin should fallen on the sides of the person So the option a) is correct.
6) An object of mass 2 kg is sliding with a constant velocity of 4 m s-1 on a frictionless horizontal table. The force required to keep the object moving with the same velocity is
a) 32 N
b) 0 N
c) 2 N
d) 8 N
Answer:- b) 0 N
No force will be required to keep the object moving with the same velocity because the sliding object has an opposing friction which help the object to stay in uniform motion.
7) Rocket works on the principle of conservation of
a) Mass
b) Energy
c) Momentum
d) Velocity
Answer:- c) Momentum
Rocket works on the principle of conservation of momentum because the gases moving out in opposite direction of the rocket helps it to move upwards with large momentum.
8) A water tanker filled up to 2/3 of its height 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
Answer:- b) move forward
Due to sudden application of the brakes, tanker stops and comes to the rest Position and the water in the tanker moves forwarded due to motion of the water.
Short Answer Questions
9) There are three solids made up of aluminium, steel and wood, of the same shape and same volume. Which of them would have highest inertia?
Answer:- Steel will have the highest inertia. As all the solids are made with same shape and same volume. The mass of the solids will play an important role in it. As the steel has maximum mass among the aluminium and wood. Thus steel would have 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.
Answer:- When the sudden brakes are applied to the train all the balls will start rotating. But they will not move with the same speed. all the balls have different mass as it is made up of different materials. As the inertia is dependent over the mass of the balls. The iron ball will have maximum inertia, and that of the plastic ball have less inertia. So the iron ball will move slowly while the rubber ball will move faster.
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?
Answer:-The light rifle will hurt the shoulder more than the heavy one.By the law of conservation of momentum, the momentum of lighter rifle is higher than the momentum of heavier rifle. Thus lighter rifle will hurt the shoulder more.
12) A horse continues to apply a force in order to move a cart with a constant speed. Explain why?
Answer:- The third Newton’s law of motion states that every action has an equal and opposite reaction . the moving cart will be opposed by the equal amount of friction by the road. Thus when the horse will overcome the opposite friction force then the cart will move with constant speed.
13) Suppose a ball of mass m is thrown vertically 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 initial and final momentums of the ball are same. Yet, it is not an example of conservation of momentum. Explain why?
Answer:-The decrease in the velocity while going upward and the Increase in the velocity while it comes downward both are due to the gravitational force of the 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.
Answer:- We have given,
m = 50 g = 50/1000 kg
Acceleration = ?
Frictional force = ?
F = m × a
F = 50/1000 × 10
F = 0.5 N
We have
Initial velocity = 0
Final Velocity = 80 m s-1
Velocity will come to rest after 8 sec of time interval due to the friction with the floor.
Now,
Acceleration = Change in velocity / Time
Acceleration = 0 -80 / 8
Acceleration = -10 m/ S2
Acceleration is negative thus, the frictional force is upon the ball is more.
Now,
F = m × a
F = 50 /1000 × (-10)
F = -0.5 N.
So the acceleration is -10 and the frictional force is -0.5 acting upon the ball.
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?
Answer:-
Given,
Mass = M
Force = F
After that,
Mass is doubled = 2m
Force is halved = f/2
Change in acceleration = ?
F = m × a
A = f/m
Calculating acceleration for changed mass and force
A* = F/2 × 2m
A* = f/4m
Thus ,
The acceleration will become 1/4th of the original acceleration.
16) Two friends on roller- skates are standing 5 m apart facing each other. One of them throws a ball of 2 kg towards the other, who catches it, how will this activity affect the position of the two ? Explain your answer.
Answer:- The activity will affect on the position between them, the separation between both of them will increase. The first when throws the ball of 2 kg to conserve the momentum he will move backwards. And another one will experience the force to catching the ball thus he will also move backwards. Thus the separation distance between both of them will increase.
17) Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. Explain the principle on the which it works.
Answer:- Water sprinkler is based upon the principal of 3rd law of motion. When the force of water exerted on the tap or nozzle of the sprinkler then the opposite force also comes there which initiates the sprinkler rotation.
Long Answer Questions
18) Using second law of motion, derive the relation between force and acceleration. A bullet of 10 g strikes a sand – bag at a speed of 103 ms-1 And gets embedded after travelling 5 cm. Calculate
i) The resistive force exerted by the sand on the bullet
Answer:-
Here we have given that,
M= 10 g
Converting M into kg = 10/1000 kg
U= 103m/s
v=O
S= 5/100 m
Now, putting into equation,
V2 – u2 = 2as
0 – ( 103)2= 2a 5/100
A = -1000×1000 / 2.5 × 10
A = 107ms-2
Now for finding force
We have an equation,
F = m× a
F = 105 N.
Thus, the resistive force exerted by the sand on the bullet is 105N.
ii) the time taken by the bullet to come to rest.
Answer:-
Time taken can be derived by the given equation-
V= u + at
So now,
0= 10 3 – 10 7 t
10 7 t = 103
T= 103/107
T = 10-4 s
Thus, the time required is 10 -4 s.
19) Derive the unit of force using the second law of motion. A force of 5 N produces an acceleration of 8 m s-2 on a mass m1 and an acceleration of 24 m s-2 on a mass m2. What acceleration would be the same force provide if both the masses are tied together?
Answer:- Here we have,
Force = mass × acceleration
F= m × a kg m s-2 or N ( Newton)
So now, we will get an equation for the M1
M1 = F / A1 = 5/8 kg.
And getting equation for M2
M2 = F/A2 = 5/24 kg
Now we have given that mass is tied together , so getting M
M = m1 + m2
M = 5/8 + 5 /24 = 5/6 kg
Now, we have to find out the acceleration produced in Mass.
So, F = 5 N given,
A = F / M
A = 5 / 5/6
A= 6 ms-2
So the acceleration will be 6 ms-2when both masses are tied together.
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.
Answer:-
The momentum is the product of mass into velocity. It is the measurement of motion of a moving body. It’s SI unit is kg m s-1.
While interpreting force in terms of momentum it will become like
The rate of changes in the momentum is the force.
A
B
Here is your solution of NCERT Exemplar Solution Class 9 Science Chapter 9 Force and Laws of Motion .
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