Welcome to the Chapter 9 - Force and Laws of Motion, Class 9 Science NCERT Solutions page. Here, we provide detailed question answers for Chapter 9 - Force and Laws of Motion. The page is designed to help students gain a thorough understanding of the concepts related to natural resources, their classification, and sustainable development.
Our solutions explain each answer in a simple and comprehensive way, making it easier for students to grasp key topics Force and Laws of Motion and excel in their exams. By going through these Force and Laws of Motion question answers, you can strengthen your foundation and improve your performance in Class 9 Science. Whether you’re revising or preparing for tests, this chapter-wise guide will serve as an invaluable resource.
Inertia is the tendency of a body to oppose the changes in their state of rest or motion unless acted upon by an external force. Inertia depends on the mass of the body, higher the mass higher will be the inertia.
a. Mass of a stone is greater than the mass of a rubber ball of the same size. Hence, the inertia of the stone is greater than that of a rubber ball.
b. Mass of a train is greater than the mass of a bicycle. Hence, the inertia of the train is greater than that of the bicycle.
c. Mass of a five rupee coin is greater than that of a one-rupee coin. Hence, inertia of the five rupee coin is greater than that of the one-rupee coin.
The velocity of the football changes four times .
(i) When the first player kicks the football, its speed changes from zero to a certain value.
(ii) When another player kicks the ball towards the goal post , the direction of the ball gets changed. Therefore, its velocity also changes.
(iii) When the goalkeeper collects the ball , the speed of the ball reduces to zero from a certain value . Hence , the velocity of the ball has changed.
(iv) The goalkeeper kicks the ball towards his team players. Hence, the speed of the ball increases from zero to a certain value. Hence, its velocity changes once again.
Agent supplying the force
In first case - first player
In second case - second player
In third case - goalkeeper
In fourth case - goalkeeper
Some leaves of the tree may get detached from a tree, if we vigorously shake its branch due to inertia of rest. Leaves are in the state of rest with respect to the tree so they possess inertia of rest. Due to this reason, the leaves fall down from the tree when shaken vigorously.
When a moving bus stops suddenly, the passengers are jerked forward because of inertia the passengers tend to remain in their state of motion even though the bus has come to rest and we fall backwards when bus starts suddenly from rest because of inertia, passengers tend to remain in state of rest though bus starts moving. Hence, the passenger tends to fall backwards when the bus accelerates forward.
To pull the cart horse pushes the ground with its foot in the backward direction by pressing the ground. As a reaction of this force, the ground pushes the horse in forward direction. Due to which the horse pulls the cart.
When a fireman holds a hose, which is ejecting large amounts of water at a high velocity, then equal and opposite reaction forces act on the fireman according to Newton’s third law. Due to this reaction force ,stability of the fireman decreases and fireman faces difficulty to hold the hose.
Mass of the rifle, m1 = 4 kg
Mass of the bullet, m2 = 50g = 0.05 kg
Recoil velocity of the rifle = v1
Bullet is fired with an initial velocity, v2 = 35 m/s
Total mass = m1 + m2 = 4 + 0.05 = 4.05 kg
Since the system was at rest , velocity (v) = 0
Momentum ( before firing ) = mass x velocity = ( m1 + m2 )v = ( 4.05 ) 0 = 0 kg m/s
Total momentum = momentum of bullet + momentum of rifle = m1 v1 + m2 v2
= 0.05 x 35 + 4 x v2 = 7/4 + 4 v2
Since the total momentum remain conserved
Total momentum before firing = Total momentum after firing
0 = 7/4 + 4 v2
4 v2 = - 7/4
v2 = - 7/16
v2 = -04375 m/s
Thus , the recoil velocity of the pistol is -0.4375 m/s .
The negative sign of velocity means that the rifle will recoil in the opposite direction of the bullet.
Mass of one of the objects, m1 = 100 g = 100/1000 = 0.1 kg
Mass of the other object, m2 = 200 g = 200/1000 = 0.2 kg
Velocity of m1 before collision, v1 = 2 m/s
Velocity of m2 before collision, v2 = 1 m/s
Velocity of m1 after collision, v3 = 1.67 m/s
Velocity of m2 after collision = v4 = ?
By law of conservation of momentum :
Total momentum before collision = Total momentum after collision
m1v1 + m2v2 = m1v3 + m2v4
(0.1)2 + (0.2)1 = (0.1)1.67 + (0.2)v4
0.4 = 0.167 + 0.2v4
v4 = 1.165 m/s
The velocity of the second object after collision 1.165 m/s.
Yes. It is possible.
(i) The object should already be moving at uniform speed in a straight line path.
(ii) There is no change in speed.
(iii) No direction change.
(iv) Friction force must be zero.
(v) Air resistance must be zero.
Since, a horizontal force of 200 N is used to move a wooden cabinet. Thus, from Newton’s third law of motion, an equal amount of force will act in the opposite direction. This opposite force is the frictional force exerted on the cabinet. Hence, a frictional force of 200 N is exerted on the cabinet.
Mass of first objects, m1 = 1.5 kg
Mass of second object, m2 = 1.5 kg
Velocity of m1 before collision, v1 = 2.5 m/s
Velocity of m2, moving in opposite direction before collision, v2 = −2.5 m/s
(Negative sign arises because mass m2 is moving in an opposite direction)
We know that
Total momentum before collision = Total momentum after collision
m1v1 + m2 v2 = (m1 + m2) v
1.5(2.5) + 1.5 (−2.5) = (1.5 + 1.5) v
3.75 − 3.75 = 3 v
v = 0 / 3 = 0
Hence, the velocity of the combined object after collision is 0 m/s.
Because of the huge mass of the truck, the force of static friction is very high. To move the car / truck, one has to apply a force more than the static friction. Therefore, when someone pushes the truck and the truck does not move, then it can be said that the applied force in one direction is cancelled out by the frictional force of equal amount acting in the opposite direction. Hence , the rationale given by the students is correct.
Mass of the hockey ball, m = 200 g = 200 / 1000 = 0.2 kg
Initial velocity, v1 = 10 m/s
Initial momentum = mv1
Final velocity, v2 = −5 m/s
Final momentum = mv2
Change in momentum = mv1 − mv2 = m(v1 - v2)
= 0.2 [10 − (−5)] = 0.2 (15) = 3.0 kg m/s
Hence, the change in momentum of the hockey ball is 3 kg m/s.
Mass of bullet, m = 10 g = 10 / 1000 = 0.01 kg
Initial velocity, u = 150 m/s
Final velocity, v = 0
Time taken, t = 0.03 s
Using the first equation of motion
v = u + at
0 = 150 + (a ×0.03 s)
(Negative sign indicates that the velocity of the bullet is decreasing.)
Now, using the third equation of motion:
v2 = u2 + 2as
0 = (150)2 + 2 (−5000) s
Hence, the distance of penetration of the bullet is 2.25 m.
We know that
Force, F = Mass × Acceleration
F = ma = 0.01 × 5000 = 50 N
Hence, the magnitude of force is 50 N.
Mass of the object, m1 = 1 kg
Mass of the wooden block, m2 = 5 kg
Velocity of the object before collision, v1 = 10 m/s
Velocity of the wooden block before collision, v2 = 0 m/s
Total mass of the combined system = m1 + m2 = 1 + 5 = 6 kg
∴ Total momentum before collision = m1 v1 + m2 v2
= 1 (10) + 5 (0) = 10 kg m s−1
Velocity of the combined object = v
Momentum = total mass x velocity of combined object
= (m1 + m2) v = 6v
According to the law of conservation of momentum:
Total momentum before collision = Total momentum after collision
m1 v1 + m2 v2 = (m1 + m2) v
10 = 6 v
The total momentum after collision is also 10 kg m/s.
Hence, velocity of the combined object after collision = 
Mass of the object, m = 100 kg
Initial velocity of the object, u = 5 m/s
Final velocity of the object, v = 8 m/s
Time take by the object to accelerate, t = 6 s
Initial momentum = mu = 100 × 5 = 500 kg m/s
Final momentum = mv = 100 × 8 = 800 kg m/s
By using Newton’s second law of motion, F = change in momentum / time taken = 
Force exerted on the object is 50 N.
According to the law of conservation of momentum, the momentum of a system before collision is equal to the momentum of the system after collision.
Hence, the change in momentum of the car and insect system is zero.
Thus, Kiran is wrong with the logic as momentum is always conserved, i.e change in momentum of insect must be equal to that of a motorcar.
Akhtar’s logic is incorrect because the mass of the car is very large as compared to the mass of the insect.
Rahul gave the correct logic as insects and motorcar experience the same force and change in momentum.
Mass of the dumbbell, m = 10 kg
Distance, s = 80 cm = 80 / 100 = 0.8 m
Acceleration, a = 10 m/s2
Initial velocity of the dumbbell, u = 0
Final velocity of the dumbbell = v
Using the third equation of motion:
v2 = u2 + 2as
v2 = 0 + 2 (10) 0.8
v = 4 m/s
Hence, the momentum with which the dumbbell hits the floor is
= mv = 10 × 4 = 40 kg m/s
When a carpet is beaten with a stick, the dust comes out of it because of the law of inertia. Initially the dust particles are at rest along with the carpet but, the dust particles try to resist their state of rest. According to Newton’s first law of motion, the dust particles stay in a state of rest, while the carpet moves. Hence, the dust particles come out of the carpet.
When a moving bus suddenly stops, the luggage on the roof tends to continue its state of motion and may fall. Also, when the bus suddenly starts from rest, luggage maintains its rest position and may fall backward. So, it is advised to tie any luggage kept on the roof of a bus with a rope.
(c) there is a force on the ball opposing the motion.
Initial velocity, u = 0
Distance travelled, s = 400 m
Time taken, t = 20 s
According to equation of motion:
a = 2 m/s2
1 metric tonne = 1000 kg (Given)
∴ 7 metric tonnes = 7000 kg
Mass of truck, m = 7000 kg
Force, F = Mass × Acceleration
F = ma = 7000 × 2 = 14000 N
The acceleration of the truck is 2 m/s2 and the force acting on the truck is 14000 N.
Mass of the stone = 1 kg
Initial velocity of the stone, u = 20 m/s
Final velocity of the stone, v = 0
Distance travelled by the stone, s = 50 m
Using the third equation of motion:
v2 = u2 + 2as
Where,
Acceleration, a
(0)2 = (20)2 + 2 × a × 50
a = −4 m/s2
Force, F = Mass × Acceleration
F = ma
F = 1 × (− 4) = −4 N
The negative sign shows that force of friction is in the opposite direction of motion , the force of friction between the stone and the ice is −4 N.
Mass of engine = 8000 kg
Mass of 1 wagon = 2000 kg
Mass of 5 wagons = 2000 x 5 = 10000 kg
Force exerted by the engine, F = 40000 N
Frictional force offered by the track, Ff = 5000 N
(a) Net force, Fa = Force of engine - Force of friction = F − Ff
= 40000 − 5000 = 35000 N
Hence, the net accelerating force is 35000 N.
(b) Total mass of the train, M = mass of 5 wagons + mass of engine
= 10000 + 8000 = 18000 kg
From Newton’s second law of motion:
Force = Mass x acceleration
35000 = 18000 x a
a = 35000 / 18000 = 1.94 m/s2
Hence, the acceleration of the train is 1.94 m/s2.
Mass of the automobile vehicle, m = 1500 kg
Acceleration of the automobile, a = −1.7 ms−2
We know that
Force = Mass × Acceleration = 1500 × (−1.7) = −2550 N
The negative sign shows that force is in the opposite direction of motion, the force between the automobile and the road is −2550 N.
(d) mv
(a) From the above table, an unequal change of distance in an equal interval of time.
Thus, the acceleration is non − uniform. Since the velocity of the object increases with time, the acceleration is increasing non-uniformly.
(b) According to Newton’s second law of motion,
Force = mass x acceleration
The force acting on an object is directly proportional to the acceleration produced in the object.
Mass of the motor car = 1200 kg
Only two persons are able to move the car with constant velocity. So, the acceleration acquired by the car is given by the third person. Thus the third person is responsible for the acceleration generated.
Acceleration, a = 0.2 m/s2
From Newton’s second law of motion:
Force = Mass × Acceleration
F = 1200 × 0.2 = 240 N
Hence, each person applies a force of 240 N to push the motor car.
Mass of the hammer, m = 500 g = 500 / 1000 = 0.5 kg
Initial velocity of the hammer, u = 50 m/s
Time taken, t = 0.01
Since the hammer comes to rest,
Velocity of the hammer, v = 0
Using Newton’s second law of motion:
The hammer strikes the nail with a force of −2500 N. By Newton’s third law of motion, nail exerts an equal force in the opposite direction on the hammer, thus the force of the nail on the hammer is equal and opposite, i.e., +2500 N.
Mass of the motor car, m = 1200 kg
Initial velocity of the motor car, u = 90 km/h = 90 x 5/18 = 25 m/s
Final velocity of the motor car, v = 18 km/h = 18 x 5/18 = 5 m/s
Time taken, t = 4 s
Using the first equation of motion:
v = u + at
5 = 25 + a (4)
a = -5 m/s2
Change in momentum = mv − mu = m (v−u)
= 1200 (5 − 25) = -24000 kg m s−1
Force = Mass × Acceleration
= 1200 × (-5) = -6000 N
Acceleration of the motor car = -5 m/s2
Change in momentum of the motor car = -24000 kg m s−1
Hence, the force required to decrease the velocity is -6000 N.
(Negative sign indicates the retardation, decrease in momentum and retarding force respectively)
(a) Both vehicles experience equal forces of action and reaction because both of the objects are moving.
(b) Since both truck and car are moving with the same magnitude of velocity v, momentum change for both vehicles is the same.
(c) Mass and acceleration are inversely proportional to each other. Therefore, car experience greater acceleration due to their smaller mass.
(d) Due to its smaller mass or opposition to the force exerted on it, the car is likely to suffer more damage than the truck.
The brakes are applied to stop the train then the train comes in the state of rest but the balls remain in the state of motion. So, due to inertia of motion, the balls move in forward direction. Since the masses of the balls are different, the balls will move with different speeds. Iron balls being heavier than the rubber ball will move with lower speed.
Assuming both the bullets have the same mass then owing to their similar velocities the recoil would be the same for both. But the lighter gun would recoil with a greater velocity (due to lower mass) while the heavier gun would attain a lower recoil velocity. So if the difference in the masses of the two guns is pretty significant then chances are that the lighter one would hurt more.
In this case the horse has to continuously exert the force to move the cart, this is because the force of friction is acting against the motion of the cart which is opposing its motion. So the horse has to apply force to overcome this force of friction.
So, the horse excerpts a constant force inorder to overcome the friction, so as to move continuously.
Momentum of a system remains conserved if no external force acts on the system. In the given case, there is a gravitational force acting on the ball which is an external force, so it is not an example of conservation of momentum.
Given, mass (m) = 50 g = 50/1000 = 0.05 kg
Initial velocity, u = 80 m/s
Final velocity, v = 0 m/s
Time taken, t = 8 sec
From first equation of motion, v = u + at
a = (v - u) / t
a = ( 0 - 80 ) / 8 = - 10 m/s^2
Now frictional force of the floor on the ball ( F ) = ma = 0.05 x (-10) = - 0.5 N (negative sign shows against the direction of motion)
Mass of truck = M, Driving force = F
Force (F) = mass (m) x acceleration (a)
Acceleration of truck = Force (F) / mass (m) = F/M
By the question, the new mass of truck = M + M = 2M
New driving force = F/2
Hence, the new acceleration = (F/2) / 2M = F/4M
Therefore, the acceleration becomes ¼ of real value.
Separation between them will increase. Initially, the momentum of both of them is zero as they are at rest. Newton’s third law of motion comes into action. In order to conserve the linear momentum, the person who throws the ball would move backward. The second will catch the ball and will move backwards i.e., in the direction of the force.
The working of the rotation of sprinklers is based on Newton's third law of motion. As the water comes out of the nozzle of the sprinkle, an equal and opposite reaction force comes into play so the sprinkle starts rotating.
Mass of bullet = 10 g = 10/1000 = 0.01 kg
Initial velocity, u = 10^3 m/s
Final velocity, v = 0 m/s
Distance, s = 5 cm = 5/100 m
Using second law of motion,
(i) v^2 - u^2 = 2 as
0^2 - (10^3)^2 = 2.a.(5/100)
a = (-1000 x 1000 x 100) / 2 x 5 = - 10^7 m/s^2
F = ma = 10^5 N
(ii) v = u + at
0 = 10^3 - 10^7 t
10^7 t = 10^3, t = 10^3 / 10^7 = 1/10^4 sec.
Time taken by the bullet to come to rest is 1/10^4 second.
According to Newton’s second law
F = ma = (kg)(m/s^2) = kgm/s^2
The absolute unit of force is called newtons (N).
m1 = F / a1 = 5/8 kg, m2 = F/a2 = 5/24 kg
If masses are tied together, then
M = m1 + m2
M = (5/8 + 5/24) = 5/6 kg
Acceleration, a = F / M = 5 / (5/6) = 6 m/s^2.
The quantity of motion possessed by a moving body is known as momentum of the body.
p = m × v
Its SI unit is kg ms-1
(a) When mass is fixed, momentum α velocity
(b) When velocity is constant, momentum α mass
Steel has the highest inertia. As mass is a measure of inertia, the solid of the same shape and size having more mass than other solids will have the highest inertia. Since steel has the greatest density and greatest mass, hence it has highest inertia.
