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Homework Help: Carnival uniform motion

  1. Sep 28, 2007 #1
    1. The problem statement, all variables and given/known data
    You've taken your neighbor's young child to the carnival to ride the rides. She wants to ride The Rocket. Eight rocket-shaped cars hang by chains from the outside edge of a large steel disk. A vertical axle through the center of the ride turns the disk, causing the cars to revolve in a circle. You've just finished taking physics, so you decide to figure out the speed of the cars while you wait. You estimate that the disk is 5 m in diameter and the chains are 6 m long. The ride takes 10 s to reach full speed, then the cars swing out until the chains are 20 from vertical.


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 28, 2007 #2

    Chi Meson

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    You've been told a few times already...

    c'mon!
     
  4. Sep 28, 2007 #3
    I am not sure what is going on
     
  5. Sep 28, 2007 #4

    Chi Meson

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    Forget the neighbor's kid. Picture a ball on a string going around an upright pole.

    Draw that picture.

    Make a free-body-diagram of the forces on the ball.
     
  6. Sep 28, 2007 #5
    Fx=Nsin(theta)=ma=m*v^2/r
    Fy=Ncos(theta)-mg=o
     
  7. Sep 28, 2007 #6

    Chi Meson

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    OK, now you need to use the information to find the tangential speed. Rotational kinematics.
     
  8. Sep 28, 2007 #7
    Using that information I get a speed of 8.93m/s which is not correct.

    v=sqrt(rgtan(theta)

    am I missing some idea
     
    Last edited: Sep 28, 2007
  9. Sep 29, 2007 #8
    it's 4.033?
     
  10. Sep 29, 2007 #9

    Chi Meson

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    I misread the problem. The radius has to be re-evaluated. Rather than a string attached to a pole, is is a string attached to a 2.5 m radius disk. The radius of the circle taken by the chair will be 2.5 m + the horizontal component of the chain's length.
     
  11. Sep 30, 2007 #10
    thanks
     
  12. Nov 14, 2007 #11
    please help

    I have done

    v=[tex]\sqrt{8.5*9.80*tan(20)}[/tex]
    = 13.65

    computer told me that it is not correct.

    where did I went wrong?

    Do I have to consider the "time factor" of 10 seconds as described in this problem?

    please help.
     
  13. Oct 22, 2008 #12
    the radius isn't 8.5, it's 2.5 plus the "horizontal component of the chain's length", which is 6sin(20).
     
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