A sphere P of mass m, travelling with speed $u$, makes a head-on collision with a stationary sphere Q also of mass m. After the collision, the velocities of P and Q are $v_1$ and $v_2$ respectively. Which one of the followings is a possible pair values for $v_1$ and $v_2$?
A. $-u, 2u$
B. $u/4, 3u/4$
C. $3u/4, u/4$
D. $u/\sqrt2, u/\sqrt2$
Using conservation of momentum, I know $v_1 + v_2 = u$.
Using energy, I know $u^2 + v^2 \le u^2$.
So I not sure which one, B or C, is the answer.