Chapter 5: Work, Energy and Power Assertion-Reason Questions | physics Class 11 CBSE

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 1

Logic Check

Assertion (A): The work done by gravity on a satellite moving in a circular orbit around the earth is zero. Reason (R): The gravitational force on the satellite is always perpendicular to its displacement in a circular orbit.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 2

Logic Check

Assertion (A): Kinetic energy of a body is always positive. Reason (R): Kinetic energy K = (1/2)mv² is the product of a positive mass and the square of speed, both of which are non-negative.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 3

Logic Check

Assertion (A): Work done by friction is always negative. Reason (R): Friction always acts opposite to the direction of motion, so the angle between friction force and displacement is always 180°.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 4

Logic Check

Assertion (A): A weightlifter holding a 150 kg mass stationary on his shoulder does no work on the load. Reason (R): Work is done only when a force causes a displacement; if displacement is zero, work done is zero regardless of force magnitude.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 5

Logic Check

Assertion (A): The total mechanical energy of a body in free fall remains constant throughout its fall. Reason (R): Gravity is a conservative force, and no non-conservative forces act on the freely falling body.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 6

Logic Check

Assertion (A): In a completely inelastic collision, the total kinetic energy of the system is conserved. Reason (R): In all collisions, the total linear momentum of the system is conserved due to Newton's third law.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 7

Logic Check

Assertion (A): The spring force is a conservative force. Reason (R): The work done by the spring force over a complete cyclic process (block returning to initial position) is zero.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 8

Logic Check

Assertion (A): The maximum speed of a block in a spring-block system occurs at the equilibrium position. Reason (R): At the equilibrium position, the spring force is zero and hence the acceleration is zero.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 9

Logic Check

Assertion (A): The work done by gravity on an object moving along any path between two points depends only on the vertical height difference and not on the path. Reason (R): Gravity is a conservative force whose work depends only on initial and final positions.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 10

Logic Check

Assertion (A): Power is a vector quantity. Reason (R): Power is defined as P = F·v, which is a scalar (dot) product of two vectors.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 11

Logic Check

Assertion (A): In an elastic collision between two equal masses where one is at rest, the first mass comes to rest and the second moves with the initial speed of the first. Reason (R): In an elastic collision, both momentum and kinetic energy are conserved; for equal masses this uniquely gives complete transfer of velocity.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 12

Logic Check

Assertion (A): When two equal masses undergo a glancing elastic collision with one at rest, they always move at right angles after the collision. Reason (R): For equal masses in an elastic collision, the dot product of the final velocity vectors v1f·v2f = 0, implying perpendicularity.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 13

Logic Check

Assertion (A): The work-energy theorem is completely independent of Newton's second law. Reason (R): The work-energy theorem is derived by integrating Newton's second law over displacement, making it a scalar integral form of the second law.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 14

Logic Check

Assertion (A): No potential energy function can be associated with the frictional force. Reason (R): The work done by friction over any closed path is negative (not zero), so friction is a non-conservative force.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 15

Logic Check

Assertion (A): The zero of potential energy is a fixed, unique physical value. Reason (R): Potential energy is defined through changes ΔV = −∫F dx; only differences in potential energy are physically meaningful, not absolute values.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 16

Logic Check

Assertion (A): During a completely inelastic collision, the loss in kinetic energy is always positive. Reason (R): The loss in kinetic energy is given by (1/2)[m1m2/(m1+m2)]v1i², which is always a positive quantity since all factors are positive.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 17

Logic Check

Assertion (A): The work done by the normal reaction force on a block sliding along an inclined plane is zero. Reason (R): The normal reaction is perpendicular to the surface, and the displacement of the block is along the surface, making the angle between them 90°.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 18

Logic Check

Assertion (A): Total linear momentum is conserved at every instant during a collision, but kinetic energy conservation applies only to the final state in an elastic collision. Reason (R): Newton's third law (which gives momentum conservation) holds at every instant, while elastic deformation energy (stored during collision) is only fully recovered after the collision ends.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 19

Logic Check

Assertion (A): A 100 W bulb operating for 10 hours consumes 1 kWh of energy. Reason (R): Energy = Power × Time = 100 W × 10 h = 1000 Wh = 1 kWh.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 20

Logic Check

Assertion (A): When a conservative force does positive work on a body, the potential energy of the body decreases. Reason (R): The change in potential energy is ΔV = −W_conservative; so if W is positive, ΔV is negative (potential energy decreases).

Reasoning Options

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Chapter sections

Chapter 5: Work, Energy and Power | Assertion-Reason

Assertion (A): The work done by gravity on a satellite moving in a circular orbit around the earth is zero. Reason (R): The gravitational force on the satellite is always perpendicular to its displacement in a circular orbit.

Assertion (A): Kinetic energy of a body is always positive. Reason (R): Kinetic energy K = (1/2)mv² is the product of a positive mass and the square of speed, both of which are non-negative.

Assertion (A): Work done by friction is always negative. Reason (R): Friction always acts opposite to the direction of motion, so the angle between friction force and displacement is always 180°.

Assertion (A): A weightlifter holding a 150 kg mass stationary on his shoulder does no work on the load. Reason (R): Work is done only when a force causes a displacement; if displacement is zero, work done is zero regardless of force magnitude.

Assertion (A): The total mechanical energy of a body in free fall remains constant throughout its fall. Reason (R): Gravity is a conservative force, and no non-conservative forces act on the freely falling body.

Assertion (A): In a completely inelastic collision, the total kinetic energy of the system is conserved. Reason (R): In all collisions, the total linear momentum of the system is conserved due to Newton's third law.

Assertion (A): The spring force is a conservative force. Reason (R): The work done by the spring force over a complete cyclic process (block returning to initial position) is zero.

Assertion (A): The maximum speed of a block in a spring-block system occurs at the equilibrium position. Reason (R): At the equilibrium position, the spring force is zero and hence the acceleration is zero.

Assertion (A): The work done by gravity on an object moving along any path between two points depends only on the vertical height difference and not on the path. Reason (R): Gravity is a conservative force whose work depends only on initial and final positions.

Assertion (A): Power is a vector quantity. Reason (R): Power is defined as P = F·v, which is a scalar (dot) product of two vectors.

Assertion (A): When two equal masses undergo a glancing elastic collision with one at rest, they always move at right angles after the collision. Reason (R): For equal masses in an elastic collision, the dot product of the final velocity vectors v1f·v2f = 0, implying perpendicularity.

Assertion (A): The work-energy theorem is completely independent of Newton's second law. Reason (R): The work-energy theorem is derived by integrating Newton's second law over displacement, making it a scalar integral form of the second law.

Assertion (A): No potential energy function can be associated with the frictional force. Reason (R): The work done by friction over any closed path is negative (not zero), so friction is a non-conservative force.

Assertion (A): The zero of potential energy is a fixed, unique physical value. Reason (R): Potential energy is defined through changes ΔV = −∫F dx; only differences in potential energy are physically meaningful, not absolute values.

Assertion (A): During a completely inelastic collision, the loss in kinetic energy is always positive. Reason (R): The loss in kinetic energy is given by (1/2)[m1m2/(m1+m2)]v1i², which is always a positive quantity since all factors are positive.

Assertion (A): The work done by the normal reaction force on a block sliding along an inclined plane is zero. Reason (R): The normal reaction is perpendicular to the surface, and the displacement of the block is along the surface, making the angle between them 90°.

Assertion (A): A 100 W bulb operating for 10 hours consumes 1 kWh of energy. Reason (R): Energy = Power × Time = 100 W × 10 h = 1000 Wh = 1 kWh.

Assertion (A): When a conservative force does positive work on a body, the potential energy of the body decreases. Reason (R): The change in potential energy is ΔV = −W_conservative; so if W is positive, ΔV is negative (potential energy decreases).

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