physics 203 help (updated)

[Riddleman]

Hi guys, lately I've been having some trouble with physics homework and was hoping you guys could help me out. These are two homework questions from my dynamics of uniform circular motion section. The first problem I got an answer them pretty confident about however the book gives me a different answer, the second problem I'm completely lost with. Any help would be appreciated. BTW, I know this isn't a physics forum, but anandtechers are about as smart as they come.
Text

---------------------------------------
(update) Here's another question from the same section that I feel bad about. The answer seems way off.
5-30

 

[Twista]

but anandtechers are about as smart as they come

<--Twista- Where the fuvk did you hear that from? :| You just offeneded all of anandtech! We are dumb.. well atleast i am

"I got lost @ Five -- 3"

 

[esun]

Well the first one is wrong because r = 0.45 m, not 0.4 m (you mixed them up in your last calculation of a = v^2/r).

You did most of the hard part of the second one. You basically found the tangential velocity of the object on the centrifuge. Now, thing about what you need to know? Basically, RPMs, which of course can be found if you know how long it takes to make one revolution. So you have v, and you have r, and in one revolution it must go a distance 2*pi*r, so t = 2*pi*r/v = 0.0057 seconds, by my calculations. So it takes 5.7 ms to finish one revolution. You have 60 seconds. Even simple dimensional analysis will give you that you should divide 60 seconds by 5.7 milliseconds per revolution (seconds / (seconds / revolution) = revolutions), so 60 / 0.0057 = 10526RPM.

 

[Riddleman]

Thanks esun, I totally see what i did wrong on the first prob, such a damn mundane error. I also get the last prob as well now, it wasn't has hopeless as i thought. Thanks again. Twista u just got proven wrong.

 

[BEL6772]

On 5-30 ... You need to re-calculate the velocity. I'm not sure what you actually typed into your calculator, but 1.03x10^22 is unreasonable.

You also need to take a little more time solving for T in your next equation. T != V/(2*pi*R).

Once you re-calculate your velocity and fix your T equation, you should be good to go ... I get 27.5 days.

 

[Twista]

Originally posted by: Gl4di4tor
Thanks esun, I totally see what i did wrong on the first prob, such a damn mundane error. I also get the last prob as well now, it wasn't has hopeless as i thought. Thanks again. Twista u just got proven wrong.

:| :roll:

hehe, so damn it be

 

[Riddleman]

Did problem 30 over and I keep getting T=4.21x10^-7 which can't be right.

 

[BEL6772]

Are you still using T = V/(2*pi*R)? If you are, you need to stop. Go back to v = 2*pi*R/T and re-solve for T.

edit: I checked your number and it looks like you're getting the right number for velocity. Just re-figure the equation for T and you're there!

 

[Riddleman]

Originally posted by: BEL6772
Are you still using T = V/(2*pi*R)? If you are, you need to stop. Go back to v = 2*pi*R/T and re-solve for T.

edit: I checked your number and it looks like you're getting the right number for velocity. Just re-figure the equation for T and you're there!

Yeah thanks i finally figured it out, the formula for time is: T = (2*pi*R)/V again another stupid mistake on my behave, I guess i have to keep practising inorder to minimize the number of errors I make. Thanks for your help, seriously appreciate it.