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Saturday, 31 October

JEE Main 2021 · PhysicsMultiple choiceDiagram basedHardDiagram reading

JEE Main 17 March 2021, Shift 1, Physics Q6

Question 6 of 90 in this shift, Physics question 6 of 30, Section A.

A mass M hangs on a massless rod of length ll which rotates at a constant angular frequency. The mass M moves with steady speed in a circular path of constant radius. Assume that the system is in steady circular motion with constant angular velocity ω\omega. The angular momentum of M about point A is LAL_A which lies in the positive zz direction and the angular momentum of M about point B is LBL_B. The correct statement for this system is :

The figure, in words

A point B at the top with a curved arrow marked ω\omega and an axis labelled zz above it; a vertical line from B down to point A, and a rod of length ll from B down to a mass M drawn as a small circle; the angle between the vertical line and the rod is labelled lpha. A dashed horizontal segment labelled r joins A to M, and a dashed circle passes through M centred on A.
  1. (1)LAL_A and LBL_B are both constant in magnitude and direction
  2. (2)LAL_A is constant, both in magnitude and directionOfficial answer
  3. (3)LBL_B is constant, both in magnitude and direction
  4. (4)LBL_B is constant in direction with varying magnitude

Official answer

Option 2

NTA final key.

Chapter
System of Particles and Rotational Motion
Topic
Angular Momentum
Idea tested
Angular Momentum of Conical Pendulum

Same idea in other shifts

Asked 2× in all
  1. 3 Sep 2020, Shift 2 · Q4A uniform rod of length 'l' is pivoted at one of its ends on a vertical shaft of negligible radius. When the shaft rotates at angular…HardDiagram basedHas 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 108 of the 174 JEE Main shifts held from 2017 to 2026.