Which of the following systems has (1) zero internal kinetic energy and (2) zero center-of-mass kinetic energy? (a) a pair of ice skaters, arms linked, skating together in a straight line; (b) a pair of skaters who start from rest facing each other and then push off so they’re moving in opposite directions; (c) a pair of skaters as in (b) but who initially are moving together along the ice before they push off
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