1. The problem statement, all variables and given/known data You slide a 2 kg block on a flat icy surface towards a static block of mass M. Before the collision, the 2 kg mass is moving at 3 m/s. The total kinetic energy is not changed by the collision. After the collision, the 2 kg mass is moving at −1 m/s i.e. opposite to its original direction. Calculate the mass M in kg and the velocity of the mass M after the collision. 2. Relevant equations Eq 1.) 0.5mv1i2+0.5Mv2i2=0.5mv1f2+0.5Mv2f2 Eq 2.) mv1i+Mv2i=mv1f+Mv2f 3. The attempt at a solution I got v2f=2 m/s and M=4 kg. I solved for M for the second equation and substituted that in into the first equation.