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Homework Help: Seemingly simple, what am i doing wrong?

  1. Oct 5, 2004 #1
    Ok, I have this problem :

    ------A device for training astronauts and jet fighter pilots is designed to rotate the trainee in a horizontal circle of radius 10.3m. If the force felt by the trainee is 7.15 times her own weight, how fast is she rotating? -------

    I already calculated that she is rotating 26.9 m/s. But now they are asking me to calculate my answer in revolutions per second. I tried using
    {2 * Pi * radius} / velocity = rpm....but I think I'm doing it wrong, since the answer wont come out right. I know that equation solves for revolutions per minute and I'm supposed to solve for revolutions per second, but I tried to multiply by 60 and it still doesn't work. Anyway, if anyone sees what I'm doing wrong, could you please tell me how to fix it and convert to rps? Thanks so much.
  2. jcsd
  3. Oct 5, 2004 #2


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    Consider (and yes, I know this is obvious - bear with me): One minute is longer than one second, so at constant speed, you'll cover more ground in a minute than in a second. That being the case, if you measure your speed in somethings/minute, it'll be a greater number than if you measure them in somethings/second. If you want to convert from revolutions/minute to revolutions/second, then, you need to divide so you end up with a smaller number than you started with.

    This can be seen more easily if you write the units as revolutions (over) minutes instead of "rpm", but I don't know the board well enough to do that here.

    Hope this helps.
  4. Oct 5, 2004 #3
    yes, you are right...that makes sense. but i did try to divide my answer by 60 and i still got the answer wrong. :/ it must be the equation i'm using in general...hmm...thanks though!
  5. Oct 5, 2004 #4


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    I'm sorry - I should've looked more closely. Consider the units of your "2*pi*radius/velocity" equation. You'll have meters on top, meters per second on the bottom. The end result will be seconds, not "rpm".

    Think about what the formula you used will give you: you're taking the length of one circumference and dividing by the speed at which you cover it. Your result is a time - what would that time represent? You're pretty close to the "revolutions per (whatever time period you're using)", but you're not quite there yet.
  6. Oct 5, 2004 #5
    ok... so 2PiR/v = T...T being the time required to travel once around the circle...but i still don't know how to go from there. Lol, sorry if this seems pathetic, I just have trouble seeing what should come next. I've tried everything I could think of...hmm...
  7. Oct 5, 2004 #6
    First of all 26.9m/s is fine and using 2 pi r is also fine

    the solution yu get for t using
    2pi r = v t should yield rev/s
    since velocity is given in m/s then the units do cancel out

    however, if the answer is still wrong ell me the answer and perhaps i can explain better for you
  8. Oct 5, 2004 #7
    I did that and got an answer of 2.405 s ...but everytime I plug it into the program, it says it's incorrect....i don't think the program is wrong, because other people solved that problem (though with different assigned numbers).
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