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Which is correct answer? How can I understand when which momentum will be conserved or not?

  • $\begingroup$ I have same doubt.. $\endgroup$ Commented Dec 28, 2017 at 6:13
  • $\begingroup$ Yes it is multi correct question.. $\endgroup$ Commented Dec 28, 2017 at 6:16
  • $\begingroup$ @XcoderX the question has asked which of the following ARE not true. This question has multiple answers. $\endgroup$
    – Mitchell
    Commented Dec 28, 2017 at 6:25
  • $\begingroup$ I see key for question and found the answer. It is c. C is wrong. But how ? $\endgroup$ Commented Dec 28, 2017 at 6:27
  • $\begingroup$ britannica.com/science/conservation-law#ref5520 should answer your query $\endgroup$
    – QuIcKmAtHs
    Commented Dec 28, 2017 at 6:29

1 Answer 1


Remember that all conservation laws always are valid. Energy is always conserved, momentum is always conserved, angular momentum is always conserved etc. If they are not valid, then we are not including the whole system.

For example, energy is of course not conserved if we allow energy to leave the system and to be absorbed by something external. Same for momentum and angular momentum.

Is momentum or angular momentum transferred out of the system in any way? This can only happen if there is some external force acting in the case of momentum or some external torque in the case of angular momentum.

So the question boils down to that: Are there external forces or torques present? Try to see if the problem can be solved through such consideration.


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