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Friday, 9 October

JEE Main 2025 · PhysicsNumerical answerNumerical valueHardMulti-step

JEE Main 7 April 2025, Shift 1, Physics Q46

Question 46 of 75 in this shift, Physics question 21 of 25, Section B.

A, B and C are disc, solid sphere and spherical shell respectively with same radii and masses. These masses are placed as shown in figure. The moment of inertia of the given system about PQ axis is x15\frac{x}{15}I, where I is the moment of inertia of the disc about its diameter. The value of xx is _________.

The figure, in words

Three touching equal-radius bodies: A (disc, shaded dark) on top, B (solid sphere, light shaded) bottom left and C (spherical shell, unshaded) bottom right; centres joined by dashed lines. Vertical axis PQ passes through the contact point between B and C and through A, with a rotation arrow at P.

Official answer

199

NTA final key.

Chapter
System of Particles and Rotational Motion
Topic
Moment of Inertia
Idea tested
MOI of Composite Bodies

Same idea in other shifts

Asked 4× in all
  1. 2 Apr 2025, Shift 1 · Q32Moment of inertia of a rod of mass 'M' and length 'L' about an axis passing through its center and normal to its length is 'α\alpha'. Now…MediumSingle correct
  2. 6 Apr 2023, Shift 1 · Q60Two identical solid spheres each of mass 2 kg and radii 10 cm are fixed at the ends of a light rod. The separation between the centres of…MediumNumerical value
  3. 28 Jul 2022, Shift 1 · Q60Four identical discs each of mass 'M' and diameter 'a' are arranged in a small plane as shown in figure. If the moment of inertia of the…MediumNumerical valueHas a figure

Question text from the official JEE Main paper published by NTA; answer from the final answer key. Chapter, topic and idea tags, difficulty and skill are Lumi’s. This question needs a figure we do not reproduce; it is described in words above. Lumi analyses 42 of the 174 JEE Main shifts held from 2017 to 2026.