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Rotation Problem

  1. Nov 7, 2005 #1
    :smile:

    The speed of a moving bullet can be deter-
    mined by allowing the bullet to pass through
    two rotating paper disks mounted a distance
    69:1 cm apart on the same axle. From the
    angular displacement 39:7 ± of the two bul-
    let holes in the disks and the rotational speed
    596 rev=min of the disks, we can determine
    the speed v of the bullet. Find the speed v of the bullet.

    Can anyone help me with this problem
     
  2. jcsd
  3. Nov 7, 2005 #2

    Fermat

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    Can you show what work you have done on this ?

    If you show us some working, we can sort out any mistakes and add suggestions.
     
  4. Nov 7, 2005 #3


    firstly, i converted the angle to radians. I then tried to find the displacement using (theta *r),............and then the velocity. But i don't think i'm using the right formula because the final answer does not look right.
     
  5. Nov 7, 2005 #4

    Fermat

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    You want to find the speed of the bullet, that is the question.

    You are given the distance between the two discs. 69.1 cm.
    All you need now is to find the time it took the bullet to pass from one disc to the next.

    What is tha angular velocity of the rotating discs ?
    What is the angular displacement the disc(s) have gone through while the bullet has passed through ?
    How much time did it take to go throught this angular displacement ?

    Hint: θ = ωt
     
  6. Nov 7, 2005 #5
    cheers



    thanks. i've worked it out. Thanks alot again. So angular displacement= angular velocity *time. And since time is scalar, its the same for the angular velocity, velocity, angular acceleration. etc. right? thank you
     
  7. Nov 7, 2005 #6

    Fermat

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    Rotational eqns of motion are very similar to linear eqns of motion.

    Linear
    s = ut + ½at²
    v² = u² + 2as
    v = u + at

    Rotational
    θ = ωt + ½αt²
    ωf² = ωi² + 2αθ
    ωf = ωi + αt
     
  8. Nov 14, 2005 #7

    that didn't answer my question, but thanks regardless
     
  9. Nov 15, 2005 #8

    Fermat

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    Sorry, skinnyabbey, bit I wasn't at all sure what your question was, so I posted some general info about rotational and linear eqns of motion.

    You said, "And since time is scalar, its the same for the angular velocity, velocity, angular acceleration. etc. right?"

    What is it that is supposed to be the same for ...
     
  10. Jan 27, 2007 #9

    Obe

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    Δθ/Δt=ω
    t=θ/ω
    t=given angle/(rpm*360 degrees/60 seconds)

    x=v(avg)t
    v=x(given)/t(solved for above)
     
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