Question

A .71-kg billiard ball moving at 2.5 m/s in the x-direction strikes a stationary ball of the same mass. After the collision, the first ball moves at 2.17 m/s, at an angle of 30.0° with respect to the original line of motion. Assuming an elastic collision (and ignoring friction and rotational motion), answer the following questions to find the struck ball's momentum after the collision.Using the conservation of momentum in the y-direction, find the struck ball's y-component of momentum.

Answer 1

After the collision, the momentum didn't change, so the total momentum in x and y are the same as the initial.

The x component was calculated by subtracting the initial momentum (total) minus the momentum of the first ball after the collision

In the y component, as at the beginning, the total momentum was 0 in this axis, the sum of both the first and struck ball has to be the same in opposite directions. In other words, both have the same magnitude but in opposite directions

$\begin{gathered} Py=0.71kg\cdot2.17\cdot sin(30) \\ Py=0.77kg\cdot m/s \end{gathered}$

This is for both balls after the collision, but one goes in a positive and the other in a negative direction.