1. The problem statement, all variables and given/known data So during collisions between 2 point masses, I know that momentum is always conserved, but energy may or may not be conserved. Now, in this case, where we have a block colliding with a pivoted rod, I know that angular momentum is always conserved, but neither energy or momentum is. Now, in the last case, if I get rid of the pivot, I believe that energy, momentum, AND angular momentum are conserved. Can somebody explain the conditions for which energy, momentum, and angular momentum is conserved? 2. Relevant equations $KE_i = KE_f$ $p_i = p_f$ $L_i = L_f$ 3. The attempt at a solution From my understanding, it's if there are no "external forces" for momentum and angular momentum. I'm not sure about energy. However, for the 2nd case, what "external force" is there? I understand there's some force from the pivot point on the rod, but why would that be "external"? And if that were considered external, why wouldn't the force from the block on the rod be considered as "external" in the 3rd case? Thanks!