1-D Collisions
In the first two cases, we can predict the final motion (solve for v1f , v2f and Elost) if we know the initial motion (m1, m2, v1i, v2i).
In the third case, we would have to know something besides the initial motion to solve for the final.
When you think about it, this does make sense, since in the third case the results of the collision should depend on the material of the colliding objects!