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JEE Main · 2014 · 06 Apr 2014 · Offline · Rotational Mechanics

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A bob of mass m attached to an inextensible string of length is suspended from a vertical support. The bob rotates in a horizontal circle with an angular speed about the vertical. About the point of suspension :
  1. (A)angular momentum changes in direction but not in magnitude.✓ Correct
  2. (B)angular momentum changes both in direction and magnitude.
  3. (C)angular momentum is conserved.
  4. (D)angular momentum changes in magnitude but not in direction.

Answer: (A)

Solution

The bob runs a horizontal circle while the string sweeps out a cone, so take moments about the fixed point of suspension. The speed and the radius stay fixed, so the size of the angular momentum, , never changes. Its direction, set by , tilts around with the string as the bob goes round, tracing its own cone about the vertical. So about the point of suspension the angular momentum keeps a constant magnitude while its direction keeps turning.

JEE Main · 2014 · 06 Apr 2014 · Offline · Single correct option

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