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Old Mar 10th 2010, 01:33 PM   #1
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confusion about how to calculate rotations when given angular velocity

In an example problem the book gives 500 rev/min. It says to solve for the number of revolutions in 5.5s you need to use the formula w=w_o+ \alpha t Why can't one just convert the revolutions into radians/second and multiply that with 5.5s? kind of like x=vt that would be the way to do it by only looking at the units of the solution

Last edited by man_in_motion; Mar 10th 2010 at 01:35 PM. Reason: removed latex b/c getting wiered latex error
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Old Mar 10th 2010, 02:23 PM   #2
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Originally Posted by man_in_motion View Post
In an example problem the book gives 500 rev/min. It says to solve for the number of revolutions in 5.5s you need to use the formula w=w_o+ \alpha t Why can't one just convert the revolutions into radians/second and multiply that with 5.5s? kind of like x=vt that would be the way to do it by only looking at the units of the solution
Please post the whole problem. How can the book gives the result in rev/min if it asks a number?
It's hard to understand where you're having problem since as stated, the problem is incomprehensible. Maybe your method works only when the angular acceleration is null and wouldn't work in the case the rotating object has an increasing angular acceleration. Just to be sure, please post the full problem.
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Old Jul 27th 2010, 12:44 PM   #3
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Cool

Originally Posted by man_in_motion View Post
In an example problem the book gives 500 rev/min. It says to solve for the number of revolutions in 5.5s you need to use the formula w=w_o+ \alpha t Why can't one just convert the revolutions into radians/second and multiply that with 5.5s? kind of like x=vt that would be the way to do it by only looking at the units of the solution
It's an old topic, thus sorry for re-opening it

Here's my attempt:

Converting 500 rev/min

= [500 X 2pi] / 60
= 52.4 radians / sec

Now Use the ratio method


1 sec = 52.4 rads
5.5 sec = x

x = 288.2 rads


Now to get your answer in rev/sec simply divide this by 2pi


= 288.2 rads / 2pi
= 45.9 revs / sec


I did whole working to make it easier for you. After you have grasped it, you can simply do the following

[500 revs / min] divided by 60

To get revs / sec

Now just multiply your answer by 5.5 secs


Hope this helps!
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