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I have observed in many experiments on collisions involving rods of varying densities that the momentum seems to first be conserved only at the point of collision then about the centre of mass. Is my reasoning correct?

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Momentum is conserved for any system whereas there are the net forces from outside of the system are zero. If you only consider a small part of the rod, it is hard to keep track of the internal forces in the rod, and you would not be able to say that the momentum is conserved. If you consider the full rod and particle system you can figure out that momentum is conserved.

Note, if the rod is attached to something, for example a wall or a hinge, we do have forces from outside of the system, and momentum will not be conserved in general.

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  • $\begingroup$ If the system consists of two massive ends joined with a light for or stick, and collision occurs at one end, will the momentum be conserved only for a mass of for both masses? I am talking about the situation just after the collision, when the internal forces haven't got enough enough time to act. $\endgroup$ – Qwerty Qwerty Jun 20 at 4:10
  • $\begingroup$ The momentum will be conserved for the full system. If there are no internal forces just after the collision, because you are using a rope instead of a stick or something, then the momentum will be conserved for one of the masses, but also for the full system! However, if the masses are connected by a stick, one often assume the internal forces to act instantly. $\endgroup$ – B. Brekke Jun 20 at 10:53

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