NCERT Solutions for Class 9th Science Chapter 8: Force and Laws Of Motion
Updated on July 13, 2025 | By Learnzy Academy
Q1. Which of the following has more inertia: (a) a rubber ball and a stone of the same size? (b) a bicycle and a train? (c) a five rupees coin and a one-rupee coin?
(a) A rubber ball and a stone of the same size
Answer: The stone has more inertia.
Reason: Inertia depends on mass. Since the stone is heavier than the rubber ball, it has more inertia.
(b) A bicycle and a train
Answer: The train has more inertia.
Reason: The train has much more mass than the bicycle, so it resists changes in motion more and has more inertia.
(c) A five-rupees coin and a one-rupee coin
Answer: The five-rupees coin has more inertia.
Reason: It has more mass than the one-rupee coin, so it has greater inertia.
Q2. In the following example, try to identify the number of times the velocity of the ball changes: “A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team”. Also identify the agent supplying the force in each case.
The velocity of the ball changes three times.
- First player kicks the football to another player – velocity changes
- Second player kicks it towards the goal – velocity changes again
- Goalkeeper kicks it towards his teammate – velocity changes once more
So, velocity changes 3 times.
Q3. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.
When we vigorously shake the branch of a tree, the branch moves suddenly, but the leaves tend to stay in their original position due to inertia. Inertia is the tendency of an object to resist any change in its state of rest or motion.
As a result, the leaves cannot follow the motion of the branch and may get detached. This is explained by Newton’s First Law of Motion, which states that an object at rest stays at rest unless acted upon by an external force.
Q4. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?
When a moving bus stops suddenly, your body keeps moving forward due to inertia, so you fall forward. When the bus starts moving from rest, your body tries to stay in the same place due to inertia, so you fall backward.
Inertia is the tendency of the body to resist any change in its state of rest or motion.
Q5. An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on
Yes, it is possible for the object to travel with a non-zero velocity even when the net external unbalanced force is zero.
Condition:
The object must be moving with a constant speed in a straight line. This means its velocity should not change.
According to Newton's First Law of Motion, an object will keep moving with the same velocity unless an unbalanced force acts on it. So, if the net force is zero, the object can keep moving at a steady speed.
Q6. When a carpet is beaten with a stick, dust comes out of it, Explain
When a carpet is beaten with a stick, the carpet moves suddenly, but the dust particles tend to stay in their original position due to inertia. Because of this, the dust gets separated from the carpet and comes out.
This happens due to Newton’s First Law of Motion, which says that an object at rest will stay at rest unless an external force acts on it. The dust remains at rest while the carpet moves, so it gets left behind.
Q7. Why is it advised to tie any luggage kept on the roof of a bus with a rope?
It is advised to tie the luggage on the roof of a bus with a rope to stop it from falling off when the bus moves suddenly, stops, or turns.
Because of inertia, the luggage tries to stay in the same position.
- If the bus starts quickly, the luggage may fall backward.
- If the bus stops suddenly, the luggage may fall forward.
- If the bus turns, the luggage may slide to the side.
Q8. A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because (a) the batsman did not hit the ball hard enough. (b) velocity is proportional to the force exerted on the ball. (c) there is a force on the ball opposing the motion. (d) there is no unbalanced force on the ball, so the ball would want to come to rest.
(c) there is a force on the ball opposing the motion.
Explanation: The ball comes to rest because of friction between the ball and the ground. This frictional force opposes the motion of the ball and slows it down until it stops.
Q9. A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration. Find the force acting on it if its mass is 7 tonnes
initial velocity (u) = 0 m/s
Distance (s) = 400 m
Time (t) = 20 s
Mass (m) = 7 tonnes = 7000 kg
Step 1: Find acceleration (a)
Use the formula:
s = ut + (1/2)at²
=> 400 = 0 × 20 + (1/2) × a × (20)²
=> 400 = (1/2) × a × 400
=> 400 = 200a
=> a = 400 ÷ 200 = 2 m/s²
Step 2: Find the force (F)
Use the formula:
F = m × a
F = 7000 × 2 = 14000 N
Hence Acceleration = 2 m/s² & Force = 14000 N
Q10. A stone of 1 kg is thrown with a velocity of 20 m s–1 across the frozen surface of a lake and comes to rest after travelling a distance of 50 m. What is the force of friction between the stone and the ice?
Mass (m) = 1 kg
Initial velocity (u) = 20 m/s
Final velocity (v) = 0 m/s (comes to rest)
Distance (s) = 50 m
Step 1: Find acceleration (a)
Use the formula:
v² = u² + 2as
=> 0 = (20)² + 2 × a × 50
=> 0 = 400 + 100a
=> 100a = -400
=> a = -400 ÷ 100 = -4 m/s²
Step 2: Find the force (F)
Use the formula:
F = m × a
F = 1 × (-4) = -4 N
Hence force of friction = 4 N (opposite to direction of motion)
Q11. A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate: (a) the net accelerating force and (b) the acceleration of the train.
Given:
Mass of engine = 8000 kg
Mass of each wagon = 2000 kg
Number of wagons = 5
Total mass of wagons = 5 × 2000 = 10000 kg
Total mass of train = 8000 + 10000 = 18000 kg
Force by engine = 40000 N
Friction force = 5000 N
(a) Net accelerating force = Force by engine − Friction force
Net force = 40000 − 5000 = 35000 N
(b) Acceleration of the train = Net force ÷ Total mass
a = 35000 ÷ 18000 = 1.94 m/s² (approximately)
Hence:-
(a) Net accelerating force = 35000 N
(b) Acceleration of the train = 1.94 m/s²
Q12. An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of 1.7 m s–2?
Mass (m) = 1500 kg
Acceleration (a) = –1.7 m/s²
Use the formula:
Force (F) = mass × acceleration
F = 1500 × (–1.7)
F = –2550 N
Hence the force between the vehicle and the road must be 2550 N opposite to the direction of motion.
Q13. What is the momentum of an object of mass m, moving with a velocity v? (a) (mv)² (b) mv² (c) ½ mv² (d) mv
(d) mv
Momentum is the product of mass and velocity.
Formula: momentum (p) = m × v
Q14. Using a horizontal force of 200 N, we intend to move a wooden cabinet across a floor at a constant velocity. What is the friction force that will be exerted on the cabinet?
When an object moves with constant velocity, the applied force is balanced by the friction force.
So, the friction force = applied force = 200 N
Q15. According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.
The student's logic is incorrect.
According to Newton’s Third Law of Motion, the action and reaction forces are equal and opposite, but they act on different objects, not on the same object. So, they do not cancel each other.
When you push the truck, the force you apply is on the truck, and the truck applies an equal and opposite force on you. These forces act on different bodies.
The truck does not move because the force you apply is not enough to overcome the friction between the truck’s tires and the ground, and the inertia of its large mass. If a much greater force is applied, the truck can move.
Q16. A hockey ball of mass 200 g travelling at 10 m/s is struck by a hockey stick so as to return it along its original path with a velocity of 5 m/s. Calculate the magnitude of change of momentum that occurred in the motion of the hockey ball due to the force applied by the hockey stick.
Mass of the ball = 200 g = 0.2 kg
Initial velocity = 10 m/s
Final velocity = -5 m/s (negative because it moves in the opposite direction)
Initial momentum = 0.2 × 10 = 2 kg·m/s
Final momentum = 0.2 × (-5) = -1 kg·m/s
Change in momentum = Final momentum - Initial momentum
= -1 - 2 = -3 kg·m/s
Magnitude of change in momentum = 3 kg·m/s
Q17. A bullet of mass 10 g travelling horizontally with a velocity of 150 m/s strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block and also calculate the magnitude of the force exerted by the block on the bullet
Mass = 10 g = 0.01 kg
Initial velocity (u) = 150 m/s
Final velocity (v) = 0 m/s
Time (t) = 0.03 s
Step 1: Find acceleration (a)
Use the formula:
v = u + at
0 = 150 + a × 0.03
a = -150 ÷ 0.03 = -5000 m/s²
Step 2: Find distance (s)
Use the formula:
s = ut + (1/2)at²
s = 150 × 0.03 + (1/2) × (-5000) × (0.03)²
s = 4.5 - 2.25 = 2.25 m
Step 3: Find force (F)
Use the formula:
F = m × a
F = 0.01 × (-5000) = -50 N
(Magnitude of force = 50 N)
Hence the distance of penetration = 2.25 m and magnitude of force = 50 N
Q18. An object of mass 1 kg travelling in a straight line with a velocity of 10 m/s collides with and sticks to a stationary wooden block of mass 5 kg. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also, calculate the velocity of the combined object.
Mass of moving object = 1 kg
Velocity of moving object = 10 m/s
Mass of wooden block = 5 kg
Velocity of wooden block = 0 m/s
Total momentum before impact = (1 × 10) + (5 × 0) = 10 kg·m/s
Let the combined velocity after impact be v
Total mass after impact = 1 + 5 = 6 kg
Using conservation of momentum:
Total momentum after impact = 6 × v
So, 10 = 6 × v
v = 10 ÷ 6 = 1.67 m/s (approximately)
Total momentum after impact = 6 × 1.67 = 10 kg·m/s
Hence -
Total momentum before impact = 10 kg·m/s
Total momentum after impact = 10 kg·m/s
Velocity of the combined object = 1.67 m/s
Q19. An object of mass 100 kg is accelerated uniformly from a velocity of 5 m/s to 8 m/s in 6 s. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.
Mass (m) = 100 kg
Initial velocity (u) = 5 m/s
Final velocity (v) = 8 m/s
Time (t) = 6 s
1.Initial momentum = mass × initial velocity
= 100 × 5 = 500 kg·m/s
2. Final momentum = mass × final velocity
= 100 × 8 = 800 kg·m/s
3. Force = rate of change of momentum
Force = (Final momentum – Initial momentum) ÷ time
= (800 – 500) ÷ 6
= 300 ÷ 6 = 50 N
Q20. Akhtar, Kiran and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar (because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation said that both the motorcar and the insect experienced the same force and a change in their momentum. Comment on these suggestions.
Rahul is correct.
According to Newton's Third Law of Motion, every action has an equal and opposite reaction. This means that the force the motorcar exerts on the insect is equal in magnitude and opposite in direction to the force the insect exerts on the motorcar.
Both the insect and the car experience the same force and the same change in momentum (but in opposite directions).
However, since the insect has a much smaller mass, it undergoes a large change in velocity and gets damaged. The car, being much heavier, shows no noticeable change in motion.
Kiran is wrong because change in momentum depends on both mass and velocity. Even though the insect’s velocity changes more, its small mass results in a smaller change in momentum.
Akhtar is also wrong because the force is not greater on the insect alone. Both the insect and the car experience the same force.
Q21. How much momentum will a dumb-bell of mass 10 kg transfer to the floor if it falls from a height of 80 cm? Take its downward acceleration as 10 m/s².
Mass = 10 kg
Height = 80 cm = 0.8 m
Acceleration due to gravity (g) = 10 m/s²
Initial velocity = 0 (since it falls from rest)
Step 1: Find final velocity before hitting the floor
Use the formula:
v² = u² + 2gh
v² = 0 + 2 × 10 × 0.8 = 16
v = √16 = 4 m/s
Step 2: Find momentum
Momentum = mass × velocity
= 10 × 4 = 40 kg·m/s
Hence the dumb-bell transfers 40 kg·m/s of momentum to the floor.