NCERT Solutions for Class 9 Science Chapter 9 Force and Laws of Motion

Q:

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 to be 10 m s−2.

A:

Q:

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.

A:

As per the conservation of momentum,

(momentum of motorcar + momentum of insect)before

 = (momentum of motorcar + momentum of insect)after

∴ Change in in momentum = 0

The direction of the insect gets reversed when it stuck on the windscreen and the velocity of the insect changes to a great extend. The motorcar continues moving with a constant velocity in the forward direction.

As, Kiran suggested that the insect suffers a greater change in momentum as compared to the motorcar hence, it is correct. The momentum of the insect after collision becomes very high as the motorcar is moving at a very high speed. Therefore, the momentum gained by the insect is equal to the momentum lost by the motorcar.

Secondly, Akhtar is also correct as the mass of the motorcar is very large as compared to the mass of the insect and the speed of the motorcar is also very high.

Now, Rahul’s explanation is correct as both the car and the insect experienced equal forces caused by the Newton’s action-reaction law. But Rahul’s statement becomes incorrect when he said that the system suffers a change in momentum because the momentum before the collision is equal to the momentum after the collision.

Q:

An object of mass 100 kg is accelerated uniformly from a velocity of 5 ms-1 to 8 ms-1 in 6 s. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.

A:

Q:

An object of mass 1 kg travelling in a straight line with a velocity of 10 ms-1 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.

A:

Q:

A bullet of mass 10 g travelling horizontally with a velocity of 150 ms−1 strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block. Also calculate the magnitude of the force exerted by the wooden block on the bullet.

A:

Q:

A hockey ball of mass 200 g travelling at 10 ms−1 is struck by a hockey stick so as to return it along its original path with a velocity at 5 ms−1. Calculate the magnitude of change of
momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.

A:

Q:

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.

A:

The static friction acting between the truck and the road is very large as the truck is very massive. Thus, truck does not move when someone pushes it. Therefore, it can be concluded that the applied force in one direction is cancelled out by the frictional force of equal amount acting in the opposite direction.

Q:

Two objects, each of mass 1.5 kg, are moving in the same straight line but in opposite directions. The velocity of each object is 2.5 ms-1 before the collision during which they stick together. What will be the velocity of the combined object after collision?

A:

Q:

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?

A:

When the force of 200 N is applied on wooden cabinet, an equal and opposite force acts on it. This opposite force is the frictional force. Therefore, a frictional force of 200 N is exerted on the cabinet.

Q:

What is the momentum of an object of mass m, moving with a velocity v?
  1. (mv)2
  2. mv2
  3. ½ mv2
  4. mv

A:

Q:

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 ms−2?

A:

Q:

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.

A:

Q:

A stone of 1 kg is thrown with a velocity of 20 ms−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?

A:

Q:

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 (Hint: 1 tonne = 1000 kg.)

A:

Q:

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.

A:

The correct option is (c)

Explanation: Here, the ball comes to rest due to frictional force which opposes its motion. Frictional force always acts in the direction opposite to the direction of motion. Therefore, this force tries to stop the ball.

Q:

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 the magnitude and direction of the velocity. If no, provide a reason.

A:

Yes. It is possible that an object is moving with a non-zero velocity even when the object experiences a net zero external unbalanced force. It is possible only when the object has been moving with a constant velocity in a certain direction. In order to change the state of motion, a net non-zero external unbalanced force must be applied on the object.

Q:

Why is it advised to tie any luggage kept on the roof of a bus with a rope?

A:

When the bus starts accelerating in forward direction, it comes in the state of motion but the luggage kept on the roof remains in the state of rest. Therefore, with the forward movement of the bus, the luggage remains at its original position and finally falls from the roof of the bus. Hence, it is advised to tie any luggage kept on the roof of a bus.

Q:

When a carpet is beaten with a stick, dust comes out of it. Explain.

A:

Due to inertia, an object opposes any change in its state of rest or state of motion. As soon as the carpet is beaten with a stick, the carpet comes into motion while the dust particles oppose their state of rest. As per Newton’s first law of motion, the dust particles remain stationary, while the carpet moves. Therefore, the dust particles come out of the carpet.

Benefits of NCERT Solutions

Following are some of the benefits of using NCERT Solutions by Extramarks.

1. They are developed by our experts with the right approach for precisely answering the textbook questions.

2. They are free of cost and available online to help class 9 students.

3. They not only explain every topic but also prepare students for further examinations. 

4. They are proven to be helpful to students in their exam preparation, homework and competitive exams.

5. They will help in understanding the solutions and the methodology of writing the answers in the exams.

Tips & Strategies for Class 9 Exam Preparation

1. Jot down important chapters or topics for your exam preparation.

2. Learn from the best teachers with Extramarks and clear your doubts instantly.

3. While practicing, refer to NCERT Solutions for class 9 to understand step-by-step solutions to all questions.

4. Solve as many sample papers as you can to prepare well for the exams.

5. Lastly, don’t panic. Try to meditate at least for 15 minutes every day- it will help you retain things better.

Why opt for Extramarks NCERT Solutions for Class 9 ?

Extramarks provide NCERT Solutions for all subjects at your fingertips. These solutions are designed by subject matter experts and provide solutions to every NCERT textbook questions. These solutions help students in their exam preparation and home assignments. We hope NCERT Solutions for class 9 will come in handy for every student and help them understand everything in a better way.

Frequently Asked Questions

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When to refer to NCERT Solutions for Class 9?

You can refer to NCERT solutions for class 9 anytime during your academic year. Whether you are preparing for class tests or unit tests or pre-board exams, you can refer to the class 9 NCERT solutions easily. The NCERT solutions are available on Extramarks - The Learning App and Extramarks website in the footer section.

What can I do to get better marks in Class 9?

If you want to gain good marks in Class 9 examination then we suggest that you go through NCERT books solutions for class 9 listed on the Extramarks website. The solutions are self-explanatory and written in easy to understand language so you can easily learn and ace your exams with the help of these solutions.

Why are the NCERT solutions Important for Class 9?

NCERT solutions class 9 are self-explanatory and form the basis of examination. Since the exam syllabus is taken directly out of the NCERT books, studying NCERT solutions becomes important to prepare for the exams.

What are some expert tips to score good marks in Class 9 CBSE?

If you want to score good marks in Class 9, follow these tips:

1. Study NCERT Solutions from Extramarks website.

2. Study using Extramarks - The Learning App, practice using the sample papers, and prepare through the interactive video modules.

3. Prepare a timetable and schedule your subjects and save time for revision.. Get concept clarity by tuning in to the Extramarks live classes.

How to Prepare for Competitive Exam Through NCERT Class 9 Solutions?

NCERT solutions for Class 9 science are amazingly useful for JEE and NEET and if you are preparing for any other competitive exam then consider studying through the NCERT solutions listed on the Extramarks website. You can also study the 9-10 and other classes syllabus on Extramarks - The Learning App.

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