I am asking about this video where The Physics Girl drops a system of balls from a height.
I am trying to estimate the velocities just after the collision using the law of conservation of momentum. And I proceed as below:
Consider the system as comprising of two balls (1: tennis ball, mass $m$, and 2: basketball, mass $M$). Now initial momentum ($p_i$) is
$$p_i = m u_1 + M u_2 = 0,$$ as the ball is dropped from rest.
The final momentum after the collision $p_f = m v_1 + M v_2.$ By law of conservation of momentum $p_i = p_f$
$$ 0 = m v_1 + M v_2 $$
$$v_1 = - (M/m)v_2 $$
Is this explanation correct? If so what is the meaning of negative sign?