# Thread: Stuck on angular motion. need to find time, and not sure what......

1. ## Stuck on angular motion. need to find time, and not sure what......

Stuck on an Angular Motion question.

Basketball is spinning on top a finger. The angular velocity of the ball slows down from 18.5 to 14.1 rad/sec. During this time, the angular displacement is 85.1 rad. Determine the time it takes for this to happen.

So theta is 85.1 rad,
for the 18.5 and 14.1 do I subtract the two? to get omega?
I am not sure and going thru my notes I can't find what I am looking for, so not sure what to do?

Which formula do I use as well. wouldn't be w=theta/time?
If someone can get me started would be great, thanks for looking.

2. Use:

$v^2 = u^2 + 2as$

to find the angular acceleration first.

$a = \dfrac{14.1^2 - 18.5^2 }{2(85.1)} = -0.843 rads^{-2}$

Now, use $v = u+at$

$18.5 = 14.1 - 0.843t$

$t = 5.22 s$

3. Oh wow, thanks. I missed the first step. I was not sure if you had to find acc or not. THANK you for the help, it helps alot.

4. You're welcome!

PHF doesn't have functional LaTeX, so I preferred answering here.