CBSE Class 9 Science Chapter 6: How Forces Affect Motion — Assertion & Reason
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The following Assertion and Reason questions are based on Chapter 6: How Forces Affect Motion from the NCERT Class 9 Science textbook. Each question has four options - read both the Assertion (A) and Reason (R) carefully before selecting your answer.
How to attempt Assertion & Reason questions:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is NOT the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is NOT the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Assertion & Reason Questions
1
Assertion (A): Force must be described using both magnitude and direction.
Reason (R): Changing either the magnitude or direction of a force can change its effect on an object.
Reason (R): Changing either the magnitude or direction of a force can change its effect on an object.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Force is a vector quantity, so a complete specification and its physical effect depend on both features.
2
Assertion (A): A spring balance can measure the magnitude of a force.
Reason (R): The SI unit of force is the newton, with symbol N.
Reason (R): The SI unit of force is the newton, with symbol N.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are correct, but the name of the unit does not explain how a spring balance measures force.
3
Assertion (A): Balanced forces produce zero net force and therefore no acceleration.
Reason (R): An object acted upon by balanced forces must immediately come to rest.
Reason (R): An object acted upon by balanced forces must immediately come to rest.
Answer: (c)
A is true, but R is false. It may remain at rest or continue with constant velocity according to its initial state.
4
Assertion (A): An unbalanced net force cannot change an object's velocity.
Reason (R): A non-zero net force produces acceleration in the direction of the net force.
Reason (R): A non-zero net force produces acceleration in the direction of the net force.
Answer: (d)
A is false, but R is true. Acceleration is precisely a change in velocity produced by an unbalanced force.
5
Assertion (A): Friction generally opposes relative motion or the tendency of relative motion between surfaces.
Reason (R): Microscopic irregularities and interactions between contacting surfaces resist sliding.
Reason (R): Microscopic irregularities and interactions between contacting surfaces resist sliding.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. These surface interactions generate a force directed against the relative motion.
6
Assertion (A): Friction is useful for walking, gripping, and braking.
Reason (R): Friction can also waste mechanical energy as heat and cause wear.
Reason (R): Friction can also waste mechanical energy as heat and cause wear.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are true, but the disadvantages do not explain its useful roles.
7
Assertion (A): Rolling friction is generally smaller than sliding friction for the same surfaces.
Reason (R): Making surfaces rougher always decreases friction between them.
Reason (R): Making surfaces rougher always decreases friction between them.
Answer: (c)
A is true, but R is false. Greater roughness commonly increases interlocking and friction rather than always reducing it.
8
Assertion (A): A backward force called inertia throws a standing passenger backward when a bus suddenly starts.
Reason (R): The passenger's body tends to retain its initial state of rest while the bus moves forward.
Reason (R): The passenger's body tends to retain its initial state of rest while the bus moves forward.
Answer: (d)
A is false, but R is true. Inertia is a property, not a backward force; the differing motion of the bus and body produces the observed effect.
9
Assertion (A): Newton's first law describes the tendency of an object to resist changes in its state of motion.
Reason (R): Without a net external force, an object at rest remains at rest and an object in motion keeps constant velocity.
Reason (R): Without a net external force, an object at rest remains at rest and an object in motion keeps constant velocity.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. The persistence of the existing state is the property called inertia.
10
Assertion (A): For a fixed mass, acceleration is proportional to the net force.
Reason (R): Mass is a scalar physical quantity.
Reason (R): Mass is a scalar physical quantity.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are correct, but the scalar nature of mass does not explain the force-acceleration proportionality.
11
Assertion (A): The same net force produces greater acceleration in a lighter object than in a heavier object.
Reason (R): For a given net force, acceleration is directly proportional to mass.
Reason (R): For a given net force, acceleration is directly proportional to mass.
Answer: (c)
A is true, but R is false. Newton's second law gives acceleration inversely proportional to mass.
12
Assertion (A): Action and reaction forces cancel because they act on the same object.
Reason (R): A Newton's-third-law pair consists of equal and opposite forces acting on two different interacting objects.
Reason (R): A Newton's-third-law pair consists of equal and opposite forces acting on two different interacting objects.
Answer: (d)
A is false, but R is true. Because the forces act on different bodies, they do not cancel in the force analysis of either single body.
13
Assertion (A): A person can walk forward by pushing the ground backward.
Reason (R): The ground simultaneously exerts a forward frictional force on the person.
Reason (R): The ground simultaneously exerts a forward frictional force on the person.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. The forward member of the action-reaction interaction accelerates the walker.
14
Assertion (A): Third-law force pairs are equal in magnitude, opposite in direction, and simultaneous.
Reason (R): Newton's first law applies when the net external force on an object is zero.
Reason (R): Newton's first law applies when the net external force on an object is zero.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both laws are stated correctly, but the first-law condition does not explain the properties of a third-law pair.
15
Assertion (A): A table moving across a floor at constant velocity has zero net horizontal force.
Reason (R): The frictional force must be greater than the applied horizontal force.
Reason (R): The frictional force must be greater than the applied horizontal force.
Answer: (c)
A is true, but R is false. For constant velocity, friction and the applied force balance in magnitude.
16
Assertion (A): Internal forces within a system can by themselves accelerate the system's centre of mass.
Reason (R): Internal action-reaction forces cancel in pairs when the entire system is considered.
Reason (R): Internal action-reaction forces cancel in pairs when the entire system is considered.
Answer: (d)
A is false, but R is true. A change in the motion of the whole system requires a net external force.
17
Assertion (A): A ball on a smooth horizontal surface continues with constant velocity when no net force acts on it.
Reason (R): Zero net force produces zero acceleration, so the velocity does not change.
Reason (R): Zero net force produces zero acceleration, so the velocity does not change.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. This is a direct application of Newton's first and second laws.
18
Assertion (A): Weight is the gravitational force exerted by the Earth on an object.
Reason (R): A liquid may exert an upward buoyant force on an immersed object.
Reason (R): A liquid may exert an upward buoyant force on an immersed object.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both force descriptions are correct, but buoyancy does not explain the definition of weight.
19
Assertion (A): An object accelerates in the direction of the net force acting on it.
Reason (R): Its velocity must always point in exactly the same direction as the net force.
Reason (R): Its velocity must always point in exactly the same direction as the net force.
Answer: (c)
A is true, but R is false. Force determines the direction of acceleration; the existing velocity can point in another direction.
20
Assertion (A): Newton's third law describes two equal and opposite forces acting on a single object.
Reason (R): Each force in a third-law pair acts on a different object.
Reason (R): Each force in a third-law pair acts on a different object.
Answer: (d)
A is false, but R is true. The two-body nature of the interaction is essential to the law.
❓ Frequently Asked Questions
What is covered in CBSE Class 9 Science Chapter 6 How Forces Affect Motion?
This chapter covers all key topics from How Forces Affect Motion as per CBSE 2026-27 syllabus.
Is this Assertion & Reason useful for CBSE board exams?
Yes, designed for CBSE Class 9 board exam preparation covering the complete syllabus.
Are these CBSE Class 9 Science Assertion & Reason updated for 2026-27?
Yes, all content at eBookPublisher is updated as per the latest 2026-27 CBSE syllabus.
How many chapters are in CBSE Class 9 Science?
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