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Circular Motion problem

  1. Dec 28, 2015 #1

    DTK

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    1. The problem statement, all variables and given/known data
    A wheel spins at an angular speed of 2.50 rad/s for a total of 30.0s. How many complete revolutions will the wheel make during this time?
    A) 31.4 rev
    B) 19.3 rev
    C) 11.9 rev
    D) 75.0 rev
    2. Relevant equations
    W=Wo+(alpha)(t)
    θ=θo+Wot+1/2(alpha)t^2
    W^2=Wo^2+2(alpha)(θ-θo)

    3. The attempt at a solution
    W=Wo+(alpha)t
    2.5rad/s=(alpha)(30s)
    Alpha=0.083

    W^2=Wo^2+2(alpha)(θ-θo)
    (2.5)^2=2(0.083)θ
    37.6=θ

    The answer I got doesnt match any of the possible answers on my worksheet so I know I'm doing something totally wrong. Please help me out this seemed pretty simple but I got stuck.
     
  2. jcsd
  3. Dec 28, 2015 #2
    What fraction of a circle is 2.5 radians?
     
  4. Dec 28, 2015 #3

    SammyS

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    There is nothing said about acceleration.

    The wheel is rotating at a constant angular (rotational) speed.
     
  5. Dec 28, 2015 #4
     
  6. Dec 28, 2015 #5

    DTK

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    I have no idea.
     
  7. Dec 28, 2015 #6
    How many degrees is 2.5 radians?
     
  8. Dec 28, 2015 #7

    DTK

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    I got 37.6=θ is that what you are asking?
     
  9. Dec 28, 2015 #8
    2.5 radians is much more than that.
     
  10. Dec 28, 2015 #9

    DTK

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    Okay how do you work it out? I obviously did it wrong. How do I get the right answer to the question
     
  11. Dec 28, 2015 #10
    Do you know how to convert radians to degrees or to revolutions?
     
  12. Dec 28, 2015 #11

    DTK

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    No
     
  13. Dec 28, 2015 #12
    Then you are not going to be able to do this problem until you do know. So go look it up in your math book or google radian.
     
  14. Dec 28, 2015 #13

    DTK

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    2.5 radian equals 143.239 degree according to google
     
  15. Dec 28, 2015 #14
    How many degrees are the in one revolution?
     
  16. Dec 28, 2015 #15

    DTK

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    1 revoltuon equals 360 degrees. 1 degree equals 0.00277 revolutions
     
  17. Dec 28, 2015 #16
    Excellent. So, based on this, how many revolutions is 143.239 degrees?
     
  18. Dec 28, 2015 #17

    DTK

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    0.3978
     
  19. Dec 28, 2015 #18
    Excellent again. So it's rotating 0.3978 revolutions every second . How many revolutions does it rotate in 30 sec
     
  20. Dec 28, 2015 #19

    DTK

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    11.9 thank you
     
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