Important Questions Class 9 Science Chapter 8: Here are the important topics to focus on from Class 9 Science Chapter 8 Force and Laws of Motion. It is important to understand the concept of force and its various types, such as balanced and unbalanced forces and how they affect the motion of objects.
The chapter covers Newton's Third Law of Motion, which explains the action-reaction principle observed in daily life situations like walking or the propulsion of a rocket. The concept of momentum and its conservation during collisions is another crucial area, with several numerical problems highlighting its application.
By practicing force and laws of motion class 9 important questions, students can strengthen their understanding of these fundamental principles.
This chapter is also referred to in class 9 science chapter 8 important questions and includes material sometimes linked with class 8 science chapter 9 important questions for broader conceptual clarity. By focusing on these topics and related questions, students can improve their problem-solving skills, especially for exams.
Important Questions Class 9 Science Chapter 8 Force and Laws of Motion
1. For every action, there is an equal and opposite reaction. This statement refers to Newton's:
(A) First Law of Motion
(B) Second Law of Motion
(C) Third Law of Motion
(D) Universal Law of Gravitation
Ans. (C)
2. An object of mass 100 kg is accelerated uniformly from a velocity of 4 m/s to 8 m/s in 2 seconds. Calculate the initial and final momentum of the object.
(A) 400 kgm/s, 800 kgm/s
(B) 100 kgm/s, 400 kgm/s
(C) 200 kgm/s, 400 kgm/s
(D) 400 kgm/s, 400 kgm/s
Ans. (A)
3. An object of mass 100 kg is accelerated uniformly from a velocity of 4 m/s to 8 m/s in 2 seconds. Calculate the magnitude of force applied.
(A) 250 N
(B) 50 N
(C) 100 N
(D) 200 N
Ans. (D)
4. A constant force acts on an object of mass 5 kg for 2 seconds, increasing its velocity from 3 m/s to 7 m/s. Find the magnitude of applied force.
(A) 5 N
(B) 10 N
(C) 15 N
(D) 20 N
Ans. (B)
5. What will happen if a balanced force is applied to a body moving with constant speed along a straight line?
(A) Speed increases
(B) Direction changes
(C) Momentum decreases
(D) Continues to move with uniform speed
Ans. (D)
6. The law of motion that defines "Force":
(A) First Law of Motion
(B) Second Law of Motion
(C) Third Law of Motion
(D) None of the above
Ans. (A)
7. How much momentum will a dumbbell of mass 10 kg transfer to the floor if it falls from a height of 80 cm? Take its downward acceleration to be 10 ms–2.
(A) 80 kg·m/s
(B) 40 kg·m/s
(C) 160 kg·m/s
(D) 0
Ans. (B)
8. A body of mass 5 kg undergoes a change in speed from 30 to 40 m/s. Its momentum would increase by:
(A) 50 kg·m/s
(B) 75 kg·m/s
(C) 150 kg·m/s
(D) 350 kg·m/s
Ans. (A)
9. The combined effect of mass and velocity is taken in account by a physical quantity called:
(A) Torque
(B) Force
(C) Inertia
(D) Momentum
Ans. (D)
10. The law which gives quantitative measure of force:
(A) First Law
(B) Second Law
(C) Third Law
(D) None
Ans. (B)
11. A vehicle of mass 120 kg is moving with a uniform velocity of 108 km/h. The force required to stop the vehicle in 10 s:
(A) 90 N
(B) 180 N
(C) 360 N
(D) 720 N
Ans. (C)
12. The rate of change of linear momentum is:
(A) Pressure
(B) Force
(C) Impulse
(D) Inertia
Ans. (B)
13. Newton's third law… two forces namely 'action' and 'reaction'. These two forces:
(A) Act on same body
(B) Act on different bodies in opposite directions
(C) Same magnitude, same direction
(D) Normal to each other
Ans. (B)
14. A man standing on a boat in still water walks towards the shore. The boat will:
(A) Move away from shore
(B) Move towards shore
(C) Stay still
(D) None
Ans. (A)
15. Walking is possible due to:
(A) First law
(B) Second law
(C) Third law
(D) None
Ans. (C)
16. A horse pulls a wagon forward because of:
(A) Force exerted by horse on ground
(B) Force exerted by ground on horse
(C) Force exerted by wagon on horse
(D) Force exerted by horse on wagon
Ans. (B)
17. A goalkeeper pulls hands backwards while holding the ball. This helps to:
(A) Exert large force
(B) Increase force exerted
(C) Increase rate of change of momentum
(D) Decrease rate of change of momentum
Ans. (D)
18. An object of mass 2 kg slides with constant velocity 4 m/s on a frictionless table. Force required to keep it moving:
(A) 32 N
(B) 0 N
(C) 2 N
(D) 8 N
Ans. (B)
19. A fielder giving a swing while catching a ball is an example of:
(A) Inertia
(B) Momentum
(C) First law
(D) Second law
Ans. (D)
20. If mass of a body is doubled and velocity becomes half, its linear momentum will:
(A) Same
(B) Double
(C) Half
(D) Four times
Ans. (A)
21. Masses of two bodies: 5:6 and velocities: 1:2. Their momentum ratio is:
(A) 5 : 6
(B) 1 : 2
(C) 12 : 5
(D) 5 : 12
Ans. (D)
22. Assertion: From Newton's second law of motion, impulse is equal to change in momentum.
Reason: Impulse and momentum have different SI units.
(A) Both assertion and reason are correct and reason is the correct explanation for assertion.
(B) Both assertion and reason are correct and reason is not the correct explanation for assertion.
(C) Assertion is correct but the reason is incorrect. (D) Both assertion and reason are incorrect.
Ans. (C)
23. Assertion: Mass is a measure of inertia of the body in linear motion.
Reason: Greater the mass, greater is the force required to change its state of rest or motion
(A) Both assertion and reason are correct and reason is the correct explanation for assertion.
(B) Both assertion and reason are correct and reason is not the correct explanation for assertion.
(C) Assertion is correct but the reason is incorrect. (D) Both assertion and reason are incorrect.
Ans. (A)
24. Assertion: If the net external force on the body is zero, then its acceleration is zero.
Reason: Acceleration does not depend on force.
(A) Both assertion and reason are correct and reason is the correct explanation for assertion.
(B) Both assertion and reason are correct and reason is not the correct explanation for assertion.
(C) Assertion is correct but the reason is incorrect. (D) Both assertion and reason are incorrect.
Ans. (C)
25. Assertion: Newton's third law applies to all types of forces. e.g. gravitational, electric or magnetic forces etc.
Reason: Newton's third law of motion is applicable only when bodies are in motion.
(A) Both assertion and reason are correct and reason is the correct explanation for assertion.
(B) Both assertion and reason are correct and reason is not the correct explanation for assertion.
(C) Assertion is correct but the reason is incorrect. (D) Both assertion and reason are incorrect.
Ans. (C)
26. Assertion: When a bullet is fired from a gun, there is a forward force on the bullet and recoil of gun. Reason: Every action has an equal and opposite reaction.
(A) Both assertion and reason are correct and reason is the correct explanation for assertion.
(B) Both assertion and reason are correct and reason is not the correct explanation for assertion.
(C) Assertion is correct but the reason is incorrect. (D) Both assertion and reason are incorrect.
Ans. (A)
27. Match the following
| Column-I | Column-II |
|---|---|
| (a) Momentum | (r) kg·m/s |
| (b) Velocity | (q) m/s |
| (c) Acceleration | (p) m/s² |
| (d) Force | (s) Newton |
Ans. (B)
28. Why does a cricket player move his hand backward while catching he ball?
Ans. A fast moving cricket ball has a large momentum. In stopping or catching this ball, its momentum has reduced to zero. Now, when a cricket player moves back his hands on catching the fast ball, then the time taken to reduce the momentum of the ball to zero is increased. Due to more time taken to stop
29. Deduce Newton's first law from second law.
30. Why do the driver and the person seated in the front seat need a seat belt?
The driver and front-seat passenger need seat belts because, during sudden stops or crashes, their bodies continue moving forward due to inertia, risking collision with the dashboard, steering wheel, or windshield. Seat belts hold them securely in place, reducing fatal injury risk by 45% for front occupants by preventing ejection from the vehicle. Without belts, they could also become projectiles harming each other. This simple restraint saves lives in all crash types.
31. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch. Ans. When the branch of the tree is shaken, the branch
Ans. When some luggage is placed on the roof of a bus which is initially at rest, the acceleration of the bus in the forward direction will exert a force (in the backward direction) on the luggage. In a similar manner, when a bus which is initially in a state of motion suddenly comes to rest due to the application of brakes, a force (in the forward direction) is exerted on the luggage. Depending on the mass of the luggage and the magnitude of the force, the luggage may fall off the bus due to inertia. Tying up the luggage will secure its position and prevent it from falling off the bus.
32. If action is always equal to the reaction, explain how a horse can pull a cart.
Ans. A horse can pull a cart because the action–reaction forces act on different bodies, so they do not cancel each other. When the horse pushes the ground backward with its hooves, the ground pushes the horse forward with an equal and opposite frictional force. This forward friction on the horse makes the horse (and so the cart attached to it) move ahead. The pull of the cart on the horse is equal and opposite to the horse’s pull on the cart, but each acts on a different object, so motion is still possible.
33. Why is luggage on a bus roof tied with rope?
Ans. Luggage on the roof of a bus is tied with a rope so that it does not slide or fall off when the bus starts, stops, or takes turns. Due to inertia, the luggage tends to remain in its previous state of rest or motion while the bus changes its speed or direction. The rope keeps the luggage fixed to the bus and prevents accidents or damage.
34. A truck travels 400 m in 20 s from rest. Calculate acceleration and force (mass = 7 tonnes).
Ans.
s = 400 m, t = 20 s, u = 0
Using s = ½at² → a = 2 m/s²
Force = ma = 7000 × 2 = 14000 N
35. A 100 kg object accelerates from 5 m/s to 8 m/s in 6 s.
Initial momentum = 500 kg·m/s
Final momentum = 800 kg·m/s
Acceleration = 0.5 m/s²
Force = 50 N
36. 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 s−210 m s−2.
