Class 11 Physics Chapter 6: Systems of Particles and Rotational Motion Assertion-reason questions with answers
2 assertion-reason questions on Systems of Particles and Rotational Motion, 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.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.
Both true. Gravity acts at the centre of mass and gives no torque about it, so Iω is conserved; tucking in lowers I, so ω rises. R explains A.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.
A is true: the force acts at the hinge line, so its line of action passes through the axis, r⊥ = 0 and the moment τ = r⊥F is zero. R is a true property of τ = r × F, but it gives only the direction of a torque; it does not explain why this torque vanishes. The zero lever arm does, so the key is (b).