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Homework Help: Angular momentum in projectile motion

  1. Jan 7, 2016 #1
    1. The problem statement, all variables and given/known data

    Is angular momentum of a projectile in projectile motion conserved ?

    2. Relevant equations

    3. The attempt at a solution
    Gravitional force vector is acting along direction of displacement vector. Thus external torque is 0. Thus angular momentum is conserved throughout projectile motion. Am i right?

    I am confused because I read somewhere that it's not.
  2. jcsd
  3. Jan 7, 2016 #2


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    Staff: Mentor

    Momentum (linear or angular) is conserved in isolated systems. That is, where no external forces or torques are applied to elements of the system, those quantities are conserved.

    In typical projectile motion problems near the surface of the Earth the gravitational force is effectively an external force acting on the projectile and we don't consider the motion of the Earth that results from the gravitational interaction. Further, the gravitational field is taken to be uniform and oriented vertically, not radially.

    So the projectile does not comprise an isolated system in such cases, and momentum (of either sort) is not conserved in such a "system".

    Why don't you set yourself a task to clear your confusion? Suppose you have a projectile which is dropped from rest from some height (pick a height). An observer at ground level and situated some distance away (pick a distance) calculates the linear and angular momentum of the projectile (from his point of view) at its initial position and just before it hits the ground. What will he find?
  4. Jan 7, 2016 #3
    This is not necessarily true. If the projectile is moving horizontally, the displacement will have a horizontal component which means the gravitational force will not be parallel to it
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