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Have you ever wondered why does a football move when you kick it, or why does it start to roll slower and finally falls when it follows the path? Or how the rotation of the steering wheel of a car alters the course of it? All of that is clarified in Chapter 8 Force and laws of motion of Class 9 Science as it aims at making students familiar with some of the fundamentals of the motion of objects and forces in their daily life.
With the help of expert teachers of Careers360, the NCERT Solutions of Class 9 chapter 8 are meant to be far more efficient and convenient to learn. The NCERT solutions also assist students in their task of learning the content of the chapter at a fast rate so that they are well prepared toward the examinations. Not only these NCERT solutions for Class 9 Science are detailed and precise in their answers to textbook questions but also they match the latest CBSE syllabus and also exam pattern quite and ideally fits well in daily studies and revision also. These solutions include answers to In-text Questions, solved Exercise Questions, Higher Order Thinking Skills (HOTS) questions to build an improved understanding, Chapter 8 Class 9 -Key Topics to learn, approach to solve questions which makes students able to solve numerical and theoretical problems efficiently.
NCERT Solutions for Class 9 Science Chapter 8 – Force and Laws of Motion offer clear, step-by-step answers to all textbook questions, making it easier for students to understand and revise key concepts. These solutions are available in a free downloadable PDF format for convenient access anytime.
Q1. Which of the following has more inertia:
(a) a rubber ball and a stone of the same size?
(c) a five rupees coin and a one-rupee coin?
Answer:
We know that inertia is defined by the mass of the body. So the body has more mass/weigth will have more inertia.
(a) In the case of rubber ball and stone of the same size, the stone will have greater mass so its inertia will be more as compared to a rubber ball.
(b) The train has much more mass as compared to the bicycle so the inertia of the train is larger.
(c) A five rupees coin has more weight than one rupee coin so the inertia of five rupees coin is more.
Q2. In the following example, try to identify the number of times the velocity of the ball changes:
Also, identify the agent supplying the force in each case.
Answer:
Velocity has both magnitude and direction. Thus change in any of magnitude or direction results in a change in velocity.
In the given case, when the player kicks the ball to another player the direction of the ball is changed thus velocity is changed.
The second player then pushes the ball in a forward direction, so velocity changes. Then the goalkeeper brings ball at rest hence velocity changes (magnitude is changed). After that he kicks the ball thus increase the speed and direction is opposite, hence velocity changes again.
The agent of supplying force is 1st player, 2nd player, goalkeeper, and goalkeeper respectively.
Q3. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.
Answer:
This can be explained with the help of the concept of inertia. When the tree is at rest, its leaves are also at rest. But when we bring the tree in motion by means of shaking it, due to the inertia of leaves they still tend to be in rest. Thus force is acting on leaves vigorously with changing direction rapidly. This results in detaching of leaves from the tree.
Answer:
In the situation of a moving bus, the passengers are also in motion along with the bus. Now when brakes are applied the bus comes to rest but due to the inertia of passengers they still tend to move in the forward direction, Thus they move in the forward direction after applying brakes.
Now when the bus is rest, the passengers are also at rest. The inertia tend them to be at rest. That's why when bus accelerates they move in a backward direction.
Q1. If action is always equal to the reaction, explain how a horse can pull a cart.
Answer:
For motion, horse applies force on the ground and thus the frictional force of the ground pushes the horse forward (action-reaction pair). Thus horse applies force on the cart and cart applies backward force on the horse but due to unbalanced force, the horse moves in the forward direction.
Answer:
This can be explained with the help of Newton's third law of motion (action-reaction pair). When the velocity of water is more i.e., water is coming out with higher force thus the reaction force on the hose is also large. This makes it difficult for a fireman to hold the hose.
Answer:
Here we will use conservation of momentum :
Initial momentum of system ( rifle + bullet ) = Final momentum of system
or
or
or
The negative sign shows that velocity is opposite to that of the bullet. This is because the force on the rifle will be opposite by bullet (action-reaction pair).
Answer:
In this question we will use conservation of momentum :
Initial momentum = Final momentum
or
or
or
Hence required velocity is 1.165 m/s.
Answer:
When a net zero external unbalanced force is applied and the object is in motion then there will be a condition in which its motion is unopposed by any external force. Then as a result object will continue to remain in motion. The velocity, in this case, will be constant and in a particular direction.
Q 2. When a carpet is beaten with a stick, dust comes out of it. Explain.
Answer:
During beating the carpet we bring it in the motion. But the dust particles have a tendency to be at rest so they resist the motion. This is why they come out of the carpet. The factor responsible for this is inertia .
Q 3. Why is it advised to tie any luggage kept on the roof of a bus with a rope?
Answer:
This is because the luggage has some mass so the inertia. When the bus is in motion, the luggage is also in motion. But when the bus stops suddenly, the luggage still tends to move with the same velocity due to inertia. Thus it may fell down. Similarly, when the bus starts from the rest, the luggage tends to be in rest thus it is pushed in a backward direction.
(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
Answer:
The frictional force present between the ground and the ball acts in the opposite direction of the motion of the ball. Thus balls stop after travelling a few distances.
Hence option (c) is correct.
Answer :
From the equations of motion we can write :
or
or
And the force acting on the truck is given by :
Answer:
By the equation of motion, we know that,
or
or
The acceleration is negative. This implies that the force is opposing the motion.
The force is given by :
or
Hence the frictional force between stone and ice is 4 N.
Answer:
The net accelerating force is given by :
Net accelerating force = Force by engine - Frictional force
Answer:
We know that the force is given by :
or
or
Thus acceleration of the train is
Answer:
By Newton's law of motion, we have :
and
or
Thus the retarding force required is 2550 N in the direction opposite to the motion of the vehicle
Q 9. What is the momentum of an object of mass m, moving with a velocity v?
Answer:
We know that the momentum of a body is given by:-
Momentum = Mass
Thus we have :
Hence option (d) is correct.
Answer:
By Newton's third law it is clear that the frictional force acting on the cabinet will be 200 N. (Note that the limit of frictional force depends upon the mass of the body and the coefficient of friction between the bodies. So it might be possible that if frictional force acting is less than 200 N).
Answer :
The student is correct. The force applied by him is less than the static friction present between tyres of the truck and the road (as the mass of truck is large thus the frictional force is also high). Thus the force applied by the student is balanced by the static frictional force which is the action-reaction pair.
Answer:
The change in the momentum is given by :
or
or
In the above equation, we have taken
Answer:
By the equation of motion we know that:-
Putting values of v, u and t in the equation :
or
or
Now we have :
or
or
Hence the distance of penetration of the bullet in the block is 2.25 m.
Now, for the force we have :
or
Thus the retarding force acting on the bullet is 50 N.
Answer:
For calculating the velocity of the combined mass we need to use the law of conservation of momentum.
or
or
or
Now, the momentum before the collision is :
or
By momentum conservation, final momentum is also
Q 15. An object of mass 100 kg is accelerated uniformly from a velocity of 5 m s-1 to 8 m s-1 in 6 s. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.
Answer:
The momentum is given by :
Initial momentum is :
And the final momentum is :
Also, the force is defined as rate of change of momentum.
Thus
or
Hence the force exerted on the body is 50 N.
Answer:
Kiran’s observation:- The insect experiences a greater change in its velocity as the mass of insects is much less as compared to the motorcar so it experiences a greater change in its momentum. Thus Kiran's observation is correct.
Akhtar’s observation:- We know that motorcar has a larger velocity and has a bigger mass as compared to the insect. Also, the motorcar continues to move in the same direction even after the collision. This suggests that the change in momentum of a motorcar is very less, whereas the insect experiences a great change in its momentum.
Rahul’s observation:- The momentum gained by the insect is equal to the momentum lost by the motorcar (law of conservation of momentum). Thus his observation is also correct.
Answer :
For calculating momentum, we need the final velocity of the bell.
By equation of motion we can write :
or
or
Thus the momentum is :
or
Time in seconds | Distance in metre |
0 | 0 |
1 | 1 |
2 | 8 |
3 | 27 |
4 | 64 |
5 | 125 |
6 | 216 |
7 | 343 |
Answer:
From the table, the relation between time and distance can be seen.
Thus the velocity of the particle is increasing with time.
and
Hence acceleration increases linearly with time.
Time in seconds | Distance in metre |
0 | 0 |
1 | 1 |
2 | 8 |
3 | 27 |
4 | 64 |
5 | 125 |
6 | 216 |
7 | 343 |
Answer:
i)Velocity is the change in distance with time. So the velocity is increasing non uniformly, so the acceleration increases with the time.
ii) Thus the net force acting on the body is also increasing.
Answer:
From the question, it is clear that two-person pushes the car with constant velocity. But when the third person pushes it has some acceleration. Thus the third person is responsible for the acceleration generated.
Force by the third person is given by :
Hence the force from each man is 240 N.
Answer:
Thus the force is given by :
or
The negative sign implies that the force by the nail on the hammer is in the opposite direction.
Answer:
The initial and final velocities are :
and
Using the equation of motion we can write :
or
or
Now the force is given by :
or
And the change in momentum is :
or
Q1:
The pulleys and strings shown in the figure are smooth and of negligible mass. For the system to remain in equilibrium, the angle θ should be
Answer:
For block A,
For block B, T-mg=m(o)
(Since both the A and B blocks are similar and the tension in the strings is mg. It has zero acceleration, as the system is in equilibrium.
Q2:
The figure shows the displacement of a particle going along the X−axis as a function of time. The force acting on the particle is zero in the region
Answer:
In the given displacement-time graph, in regions AB and CD, the graph is linear i.e. velocity (slope) is uniform i.e., constant and we know that acceleration = change in velocity/time. Since, velocity is constant so change in velocity is 0 and acceleration is 0.
From Newton's second law of motion,
And F = m × a = 0
Q3:
Two particles of mass m each are tied at the ends of a light string of length 2a. The whole system is kept on a frictionless horizontal surface with the string held tight so that each mass is at a distance a from the center P (as shown in the figure). Now, the midpoint of the string is pulled vertically upwards with a small but constant force F. As a result, the particles move towards each other on the surface. Neglecting the earth's gravitation, the magnitude of acceleration, when the separation between them becomes 2x, is :
Answer:
The acceleration of mass m is due to the force
$
\begin{aligned}
& \therefore T \cos \theta=m a \Rightarrow a=\frac{T \cos \theta}{m} \ldots \text { (i) } \
& \text { also, } F=2 T \sin \theta \Rightarrow T=\frac{F}{2 \sin \theta} \ldots \text { (ii)
\end{aligned}
$
From(i)and(ii)
Q4:
There are cars with masses of 4 kg and 10 kg respectively that are at rest. A car having a mass of 10 kg moves towards the east with a velocity of 5 m/s. Find the velocity of the car with a mass of 4 kg with respect to the ground.
Answer:
Given,
m1 = 4 kg
m2 = 10 kg
v1 = ?
v2 = 5 m.s-1
We know from the law of conservation of momentum that,
pinitial = 0, as the cars are at rest
pfinal = p1 + p2
pfinal = m1.v1 + m2.v2
pfinal = 4 kg.v1 + 10 kg.5 m.s-1
Pi = Pf
0=4 kg.v1+50 kg.m.s-1
v1 = 12.5 m.s-1
Q5:
A body of mass 300g kept at rest breaks into two parts due to internal forces. One part of mass 200 g is found to move at a speed of 12 m/s towards the east. What will be the velocity of the other part?
Answer: Before it broke, the body was at rest. The linear momentum of the body was thus
Force and Laws of Motion cover all the key topics that explain how forces affect the motion of objects, including Newton’s laws, inertia, and momentum. These topics form the foundation of classical mechanics and are essential for understanding real-world motion.
8.1 Balanced And Unbalanced Forces
8.2 First Law Of Motion
8.3 Inertia And Mass
8.4 Second Law Of Motion
8.4.1 Mathematical Formulation Of Second Law Of Motion
8.5 Third Law Of Motion
Here are the important formulae from Class 9 Science Chapter 8 – Force and Laws of Motion, which are essential for solving numerical problems and understanding concepts:
Where:
Where:
Where:
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The study of force enables us to understand why and how things move or remain stationary. It's the basis for the study of motion and mechanics.
Equilibrium in forces results in no change in the movement of an object, but unbalanced forces result in a change in movement or direction.
Due to inertia, the lower body which is in contact with the bus does not move anymore, but the upper body still shows signs of movement, and the person finds himself falling forward.
Due to its high inertia. Heavy bodies resist motion changes more than light bodies.
Yes, if it was already uniformly moving, it will keep on moving with the same velocity. Equilibrium forces do not produce acceleration.
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