# Could you help me review and check my physics homework?

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• Oct 16th 2012, 08:43 AM
Glyper
Re: Could you help me review and check my physics homework?
Turned out the task said it was r=0.1, not d=0.1, my bad ;) Then, we have 3130rad/s which is around 31300rpm, right?

Also, I spotted that in 3. there IS a mistake - namely, I'm using 150cm (the diameter) as the radius which ruins the final answer. It should be cirmuference = 471cm and then 20.1rad/s as a consequence, shouldn't it?
• Oct 16th 2012, 10:38 AM
ebaines
Re: Could you help me review and check my physics homework?
Quote:

Originally Posted by Glyper
Turned out the task said it was r=0.1, not d=0.1, my bad ;) Then, we have 3130rad/s which is around 31300rpm, right?

To convert radians/second to RPM you need to multiply by 60 seconds/minute and divide by 2 $\displaystyle \pi$ radians per revolution. You should get an answer a bit less than 30,000 RPM

Quote:

Originally Posted by Glyper
Also, I spotted that in 3. there IS a mistake - namely, I'm using 150cm (the diameter) as the radius which ruins the final answer. It should be cirmuference = 471cm and then 20.1rad/s as a consequence, shouldn't it?

I get precisely 20 rad/s. An easy way to do this is to use the fact that liinear velocity and rotational velocity are related by $\displaystyle v=\omega R$. Here $\displaystyle v= 15 m/s$ and $\displaystyle R= 0.75m$, so

$\displaystyle \omega = \frac {15 \frac m s}{0.75 m} = 20/s$
• Oct 16th 2012, 11:49 AM
Glyper
Re: Could you help me review and check my physics homework?
OK, I think I get it. Thank you a lot for all your help :)
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