systems can be defined and redefined and so the meaning of what an internal or an external force is also changes: I think im wrong here: if then we consider planet earth a system, all the things ( cars moving etc ) that happen in this system, are they internal forces,?
if there is a change in momenta inside the system, then it is caused by a force. no doubt. internal forces. why then is it said that an internal force cant cause a change in momentum?
what i think: the system's momentum as a whole doesnt change .. but i cant go anywhere from that fact for some reason. im stuck
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