Physics · Work, energy and power · chapter 5 of 14

Class 11 Physics Chapter 5: Work, Energy and Power Assertion-reason questions with answers

2 assertion-reason questions on Work, Energy and Power, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): The work done by gravity on a stone carried from the foot of a hill to its top is the same whether it is taken up a steep path or a gentle winding road. Reason (R): Gravity is a conservative force, so the work it does depends only on the initial and final positions.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both are true. The work done by a conservative force depends only on the end points, not on the path; that is exactly why the two paths give the same work by gravity. R explains A.

Question 2
Assertion (A): In an elastic collision of two bodies, the total kinetic energy after the collision equals the total kinetic energy before it. Reason (R): The total linear momentum of the two bodies is conserved in every collision.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Both are true. A is the definition of an elastic collision; R holds for elastic and inelastic collisions alike, from Newton's third law. Momentum conservation does not imply kinetic energy conservation, so R does not explain A.

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