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Question 30379
JEE Physics · Hard

Rigid Body Dynamics: Advanced — Variable I MCQ

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Advanced — Variable IHardQuestion 30379

Question

A dumbbell (two masses m, separated by massless rod) rotates about its centre. If the rod is slowly doubled in length while spinning freely, the KE:
  1. A
    Doubles
  2. B
    Quadruples
  3. C
    Becomes one quarter
    Correct
  4. D
    Halves

Correct answer

Becomes one quarter

Explanation

Each mass at 2× radius: I_new = 4×I_old. By L conservation: L_new = L_old. KE = L²/(2I): KE_new = L²/(2×4I) = KE_old/4.

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