Two skaters, at rest and facing each other, push off each other. Their initial velocities and momenta are zero, but their final momenta are not. How can
the total initial momentum (which is zero) equal the total final momentum,
since both skaters are now moving? (Remember that momentum is a vector.)
A. This is not an isolated system, so the conservation law does not
apply.
O B. Momentum is not conserved in this case.
O C. The skaters are moving in opposite directions. One momentum is
positive and the other negative, so their values add up to zero.
O D. One skater must be much larger than the other.