CBSE Class 9 Science Chapter 7: Work, Energy, and Simple Machines — Assertion & Reason
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The following Assertion and Reason questions are based on Chapter 7: Work, Energy, and Simple Machines 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): A constant force does positive work when it produces displacement in its own direction.
Reason (R): For parallel force and displacement, work equals force multiplied by displacement.
Reason (R): For parallel force and displacement, work equals force multiplied by displacement.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. The aligned components give W = Fs and a positive value.
2
Assertion (A): Work and energy are both measured in joules in the SI system.
Reason (R): Energy is the capacity to do work.
Reason (R): Energy is the capacity to do work.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are true, but the definition of energy does not by itself explain the shared SI unit.
3
Assertion (A): A person carrying a load horizontally at constant height does zero work on it through the upward supporting force.
Reason (R): The displacement of the load is vertically upward in the direction of that supporting force.
Reason (R): The displacement of the load is vertically upward in the direction of that supporting force.
Answer: (c)
A is true, but R is false. The horizontal displacement is perpendicular to the upward force, so the work by that force is zero.
4
Assertion (A): A force opposite to an object's displacement does positive work on the object.
Reason (R): A force opposite to displacement does negative work.
Reason (R): A force opposite to displacement does negative work.
Answer: (d)
A is false, but R is true. The angle of 180 degrees makes the force component along displacement negative.
5
Assertion (A): The net work done on an object equals its change in kinetic energy.
Reason (R): A positive net work increases kinetic energy, while negative net work decreases it.
Reason (R): A positive net work increases kinetic energy, while negative net work decreases it.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. This consequence directly expresses the work-energy theorem.
6
Assertion (A): An object that has the capacity to do work possesses energy.
Reason (R): Kinetic energy is the energy an object has because of its motion.
Reason (R): Kinetic energy is the energy an object has because of its motion.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both are correct, but one example of energy does not explain the general definition.
7
Assertion (A): The kinetic energy of an object is one-half its mass multiplied by the square of its speed.
Reason (R): Doubling the speed of an object of fixed mass only doubles its kinetic energy.
Reason (R): Doubling the speed of an object of fixed mass only doubles its kinetic energy.
Answer: (c)
A is true, but R is false. Because speed is squared, doubling it makes kinetic energy four times as large.
8
Assertion (A): A stretched rubber band at rest has no energy because its kinetic energy is zero.
Reason (R): Deformation can store elastic potential energy even when an object is motionless.
Reason (R): Deformation can store elastic potential energy even when an object is motionless.
Answer: (d)
A is false, but R is true. Zero kinetic energy does not imply zero total energy.
9
Assertion (A): Near the Earth's surface, gravitational potential energy can be written as mgh.
Reason (R): Raising an object increases the energy associated with its position in the Earth-object system.
Reason (R): Raising an object increases the energy associated with its position in the Earth-object system.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Work against gravity is stored as gravitational potential energy.
10
Assertion (A): At the highest point of a vertically thrown ball, its instantaneous kinetic energy is zero.
Reason (R): Its acceleration at that point remains directed downward due to gravity.
Reason (R): Its acceleration at that point remains directed downward due to gravity.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are true, but the continued acceleration does not explain the instantaneously zero speed.
11
Assertion (A): Mechanical energy remains constant when only conservative forces act and dissipative effects are absent.
Reason (R): In the presence of friction, the sum of kinetic and potential energy must still remain unchanged in every case.
Reason (R): In the presence of friction, the sum of kinetic and potential energy must still remain unchanged in every case.
Answer: (c)
A is true, but R is false. Friction converts part of the mechanical energy into thermal and other forms.
12
Assertion (A): Friction destroys energy and violates the law of conservation of energy.
Reason (R): Friction transforms organised mechanical energy into thermal energy and other forms.
Reason (R): Friction transforms organised mechanical energy into thermal energy and other forms.
Answer: (d)
A is false, but R is true. Total energy is conserved even though mechanical energy may decrease.
13
Assertion (A): Power is the rate at which work is done.
Reason (R): The same amount of work completed in less time requires greater average power.
Reason (R): The same amount of work completed in less time requires greater average power.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Since P = W/t, reducing time for fixed work increases power.
14
Assertion (A): Two machines can do the same work while operating at different powers.
Reason (R): The joule is the SI unit of work.
Reason (R): The joule is the SI unit of work.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are correct, but the unit of work does not explain the difference in completion rates.
15
Assertion (A): One watt is equal to one joule per second.
Reason (R): A machine doing 100 J of work in 20 s has a power of 20 W.
Reason (R): A machine doing 100 J of work in 20 s has a power of 20 W.
Answer: (c)
A is true, but R is false. Its power is 100/20 = 5 W.
16
Assertion (A): An ideal simple machine reduces the total amount of work required for a task to zero.
Reason (R): A simple machine can reduce the required effort by increasing effort distance or changing the force direction.
Reason (R): A simple machine can reduce the required effort by increasing effort distance or changing the force direction.
Answer: (d)
A is false, but R is true. Machines trade force for distance or convenience; they do not eliminate work.
17
Assertion (A): A fixed pulley can make lifting more convenient by changing the direction of the applied force.
Reason (R): Pulling downward on the rope produces an upward force on the load through the pulley system.
Reason (R): Pulling downward on the rope produces an upward force on the load through the pulley system.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. The direction change lets a person use a more convenient downward effort.
18
Assertion (A): A movable pulley can reduce the effort required to lift a load.
Reason (R): An inclined plane can also reduce effort by increasing the distance over which the force acts.
Reason (R): An inclined plane can also reduce effort by increasing the distance over which the force acts.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both machines can provide force advantage, but the inclined-plane principle does not explain the operation of the movable pulley.
19
Assertion (A): A lever's mechanical advantage depends on the relative lengths of the effort arm and load arm.
Reason (R): In every lever, the load must lie between the fulcrum and the effort.
Reason (R): In every lever, the load must lie between the fulcrum and the effort.
Answer: (c)
A is true, but R is false. The positions vary among the three classes of levers.
20
Assertion (A): An inclined plane makes the work needed to raise a load equal to zero.
Reason (R): It allows a smaller force to act through a longer distance to achieve the same height gain.
Reason (R): It allows a smaller force to act through a longer distance to achieve the same height gain.
Answer: (d)
A is false, but R is true. The machine changes the force-distance combination rather than eliminating the energy requirement.
❓ Frequently Asked Questions
What is covered in CBSE Class 9 Science Chapter 7 Work, Energy, and Simple Machines?
This chapter covers all key topics from Work, Energy, and Simple Machines 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?
All chapters of CBSE Class 9 Science are covered at eBookPublisher with free Assertion & Reason for each chapter.
Can I study Work, Energy, and Simple Machines online for free?
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