1. The problem statement, all variables and given/known data Assume an elastic collision (ignoring friction and rotational motion). A queue ball initially moving at 3.6 m/s strikes a stationary eight ball of the same size and mass. After the collision, the queue ball’s final speed is 1.9 m/s . Find the queue ball’s angle θ with respect to its original line of motion. Answer in units of ◦. 2. Relevant equations p1x + p2x = p1x' + p2x' p1y + p2y = p1y' + p2y' p = mv 3. The attempt at a solution OK I got the right answer (really I just copied my teacher's work which was too confusing for me to repeat, I only plugged in numbers to get the right answer), which was about 58.14 degrees. Really don't feel like typing out the whole process, but she (the teacher) used the sin^2 theta + cos^2 theta = 1 identity during the process. THEN! My friend shows me how she did it, and all she did was arccosine (final speed of ball 1/initial speed of ball 1), which got the exact same answer. I don't get the physics behind this. Why did my friend's solution work?