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Clarification about conservation of Energy/Momentum

  1. Nov 5, 2013 #1

    I'm having some difficulties keeping when Linear/Angular Momentum and Kinetic Energy are conserved straight.

    So far I believe that Kinetic energy is conserved when no outside/non-conservative forces are acting on an object. It is conserved only in perfectly elastic collisions, which only occur at the atomic level(?) Then it decreases in inelastic collisions and increases in explosions.

    Linear momentum is conserved when Kinetic Energy is conserved, ie, no outside forces acting on the object and in all types of collisions.

    Angular momentum is conserved when the net torque acting on an object is 0.
    Here's my big question on this one, how do I know when the net torque is 0? I believe that torque is the tendency of an object to rotate, so when there's no rotation is there no torque?

    I just need a little bit of clarification on all of these.

    Thank you so much for your help!
  2. jcsd
  3. Nov 5, 2013 #2


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    Staff: Mentor

    That's not true. Kinetic energy for a single particle without any internal structure is conserved if no force at all is acting on it, but that is a boring situation, the particle will go in a straight line.
    Kinetic energy is also conserved in elastic collisions, without any additional forces.
    Let's say atoms are a very common place to find elastic collisions. It is not exclusive, and not all atomic collisions are elastic, but that is a good approximation.
    That's way too narrow. Linear momentum is conserved when there is no outside force. It does not matter what happens internally.

    Calculate torque, see if it zero.

    No (and it would not even follow from the above statement if that would be true).
    In particular, if you have a net linear force, there are always reference frames where the torque is not zero.
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