(adsbygoogle = window.adsbygoogle || []).push({}); 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?

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# Homework Help: 2D momentum (need help in understanding, but have the answer)

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