CBSE Class 9 Science Exemplar Chapter 9: Force and Laws of Motion NCERT Solutions
This resource provides detailed NCERT Solutions for Class 9 Science Exemplar, Chapter 9: Force and Laws of Motion. It covers fundamental concepts including Newton's three laws of motion, inertia, acceleration, and the relationship between force, mass, and momentum. The solutions explain how these principles apply in everyday scenarios, such as a goalkeeper saving a goal or the motion of a train. This chapter is crucial for understanding the basic principles governing the movement of objects. These solutions are designed to help students grasp complex ideas, clarify doubts, and prepare effectively for their examinations by offering step-by-step explanations and clear reasoning for each problem.
Quick info
| Board | CBSE |
|---|---|
| Class | Class 9 |
| Subject | Science Exemplar |
| Session | 2026 |
| Language | English |
| Type | NCERT Solutions |
| Chapter | Chapter 9 |
Chapter summary
Chapter 9 of the Class 9 Science Exemplar focuses on the fundamental principles of Force and Laws of Motion. The NCERT Solutions provided here cover key concepts such as inertia, the three laws of motion, and the effect of forces on objects. The exercises include multiple-choice questions and problems that require understanding the application of these laws in various physical situations, emphasizing the relationship between force, mass, velocity, and acceleration.
Learning outcomes
- Understand the concept of inertia and its relation to the state of motion.
- Apply Newton's third law of motion to analyze action-reaction pairs.
- Explain how forces affect the motion of objects, including acceleration.
- Analyze scenarios involving changes in momentum and the forces causing them.
- Solve problems related to uniform and accelerated motion.
- Differentiate between accelerated, uniform, and retarded motion.
Topics covered
Paper topics
- Force and Motion
- Newton's First Law of Motion
- Inertia
- Mass and Inertia
- Newton's Second Law of Motion
- Momentum
- Newton's Third Law of Motion
- Action and Reaction Forces
- Acceleration
- Uniform Motion
- Accelerated Motion
- Frictionless Surfaces
Important topics
- Newton's Laws of Motion
- Inertia and its types
- Concept of Force
- Momentum and its change
- Action-Reaction Principle
- Distinguishing between uniform and accelerated motion
PDF preview
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Questions and Solutions
Multiple Choice Questions - 1
(a) Its speed keeps changing
(b) Its velocity always changes
(c) It always goes away from the earth
(d) A force is always acting on it
Explanation: An object undergoing accelerated motion along a straight path implies that its velocity is continuously changing. This change in velocity means its speed is also changing, and a net force must be acting on it according to Newton's second law. However, the direction of motion is not necessarily away from the Earth. For an object to move away from the Earth, its acceleration must overcome gravitational pull, and simply moving in a straight line with acceleration is not sufficient. Therefore, the statement that it always goes away from the Earth is incorrect.
Multiple Choice Questions - 2
(a) always act on the same body
(b) always act on different bodies in opposite directions
(c) have same magnitude and directions
(d) act on either body at normal to each other
Explanation: Newton's third law of motion states that for every action, there is an equal and opposite reaction. These action and reaction forces always occur in pairs. Crucially, they act on different bodies. For example, when you push a wall (action), the wall pushes back on you (reaction). These forces are equal in magnitude and opposite in direction, but they do not cancel each other out because they are applied to different objects.
Multiple Choice Questions - 3
(a) exert larger force on the ball
(b) reduce the force exerted by the ball on hands
(c) increase the rate of change of momentum
(d) decrease the rate of change of momentum
Explanation: When the goalkeeper pulls his hands backward after catching the ball, he increases the time interval during which the ball's momentum changes from its initial value (when it was shot) to zero (when it's held). According to the impulse-momentum theorem, the impulse (which is equal to the force multiplied by the time interval) is equal to the change in momentum. By increasing the time, the average force exerted by the ball on the goalkeeper's hands is reduced, making it less painful and easier to handle.
Multiple Choice Questions - 4
(a) to increase its speed
(b) to decrease its speed
(c) to resist any change in its state of motion
(d) to decelerate due to friction
Explanation: Inertia is the fundamental property of matter that resists any change in its state of motion. An object at rest tends to stay at rest, and an object in motion tends to stay in motion with the same speed and in the same direction, unless acted upon by an external force. Inertia itself does not cause an object to increase or decrease its speed, nor is it directly related to deceleration due to friction, although friction can be a force that overcomes inertia.
Multiple Choice Questions - 5
(a) accelerated
(b) uniform
(c) retarded
(d) along circular tracks
Explanation: When a passenger tosses a coin, the coin initially shares the motion of the train. If the train were moving at a uniform velocity, the coin would land back in the passenger's hand. If the train were decelerating (retarded), the coin would land ahead of the passenger. Since the coin falls behind the passenger, it indicates that the train's velocity is increasing, meaning the train is accelerating forward. The coin, due to inertia, tends to maintain its previous velocity, while the train moves forward faster.
Multiple Choice Questions - 6
(a) 32 N
(b) 0 N
(c) 2 N
(d) 8 N
Explanation: According to Newton's first law of motion, an object in motion will continue in motion with a constant velocity unless acted upon by a net external force. In this scenario, the object is already moving at a constant velocity of 4 m/s on a frictionless horizontal table. Since there is no friction and no other horizontal force is mentioned, the net force acting on the object is zero. Therefore, no additional force is required to maintain this constant velocity.
Common mistakes
- Confusing action and reaction forces as acting on the same body.
- Misinterpreting inertia as a force that causes motion.
- Incorrectly applying the concept of force in the absence of acceleration (e.g., uniform motion).
- Not understanding that a change in the rate of momentum implies a force.
Revision tips
- Review Newton's three laws of motion and their real-world examples.
- Focus on understanding the concept of inertia and how it resists changes in motion.
- Practice identifying action-reaction pairs correctly.
- Analyze the provided explanations for each question to reinforce understanding of the underlying principles.
Practice MCQs
Q1. Which statement is NOT correct for an object moving along a straight path in an accelerated motion?
Explanation: An object in accelerated motion along a straight path will have a changing velocity and speed, and a net force acting on it. However, it does not necessarily move away from the Earth; its motion depends on the direction and magnitude of acceleration.
Q2. According to Newton's third law of motion, action and reaction forces:
Explanation: Newton's third law states that for every action, there is an equal and opposite reaction. These forces always act on different bodies, are equal in magnitude, and opposite in direction.
Q3. A goalkeeper pulls his hands backward after catching a football shot. This action helps to:
Explanation: By pulling his hands backward, the goalkeeper increases the time over which the ball's momentum changes. According to the impulse-momentum theorem (Force × time = change in momentum), increasing the time reduces the average force exerted on his hands.
Q4. The inertia of an object primarily tends to cause the object to:
Explanation: Inertia is the inherent property of an object to resist any alteration in its state of motion (whether at rest or in uniform motion). It does not inherently cause changes in speed or deceleration due to friction.
Q5. If a passenger in a moving train tosses a coin that falls behind him, it implies that the train's motion is:
Explanation: If the train were moving uniformly, the coin would land back in the passenger's hand. If it were retarded, the coin would fall ahead. Falling behind indicates the train is accelerating forward, meaning its velocity is increasing.
Q6. An object of mass 2 kg is sliding with a constant velocity of 4 m/s on a frictionless horizontal table. What is the net force required to keep it moving with the same velocity?
Explanation: According to Newton's first law of motion, an object in motion will stay in motion with constant velocity unless acted upon by a net external force. Since the table is frictionless and the velocity is constant, the net force required is 0 N.
Frequently asked questions
What is the main focus of Chapter 9, Force and Laws of Motion, for Class 9 Science?
Chapter 9 focuses on the fundamental principles governing the motion of objects, including inertia, Newton's three laws of motion, the concept of force, momentum, and acceleration.
How do these NCERT Solutions help in understanding inertia?
The solutions explain inertia as the resistance to changes in the state of motion and provide examples to illustrate how it affects objects, helping students grasp this key concept.
What is the significance of Newton's third law as explained in these solutions?
The solutions clarify that Newton's third law states action and reaction forces are equal in magnitude, opposite in direction, and act on different bodies, which is crucial for analyzing interactions.
How are real-world examples used in the solutions?
Real-world scenarios like a goalkeeper catching a ball or a moving train are used to demonstrate the application of the laws of motion, making the concepts more relatable and easier to understand.
Are these solutions useful for exam preparation?
Yes, these solutions provide clear, step-by-step explanations for each question, helping students build a strong foundation and revise the chapter effectively for exams.
What is the difference between uniform and accelerated motion, according to the chapter?
Uniform motion implies constant velocity (no net force), while accelerated motion involves a change in velocity (speed or direction) due to a net force acting on the object.
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