A small marble with mass m = 18 g is traveling with speed
Vi= 4.0 m/s across a frictionless surface. Then it under-
goes an elastic, head-on collision with a larger marble that
is initially at rest. The smaller marble recoils backward
with speed vf = 3.1 m/s. What is the mass M of the
larger marble? The symbolic answer should be stated in
terms of m, vi, and vf, though not all of these quantities
may be needed.