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Which of the following quantity remains conserved when the moon revolves around the earth?
1. Velocity
2. Linear momentum
3. Angular momentum
4. All of these

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Correct Answer - Option 3 : Angular momentum


Angular momentum:

  • Angular momentum is the property of any rotating object given by moment of inertia times angular velocity.
    • It is a vector quantity.
    • Its SI unit is kg-m2/sec.
  • If I and ω are the moment of inertia and the angular velocity respectively, then the angular momentum is given as,

⇒ L = Iω

⇒ L = rP

Where r = radius of rotation and P = linear momentum

Conservation of angular momentum:

  • According to the conservation of angular momentum, if the total external torque on the system of particles is zero, then the total angular momentum of the system remains conserved.


  • When the moon revolves around the earth, its direction of motion continuously changes so the velocity and the linear momentum also change continuously.
  • Since no torque acts on the moon when it revolves around the earth, so according to the conservation of angular momentum the angular momentum of the moon will remain conserved. Hence, option 3 is correct.

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