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Homework Help: Ferris Wheel -

  1. Jun 3, 2007 #1
    1. The problem statement, all variables and given/known data
    How many revolutions per minute would a 15m diameter Ferris wheel need to make for a person to feel 'weightless' at the topmost point?

    2. Relevant equations
    F= mv~2/r - mg
    v = 2pi r/ T

    3. The attempt at a solution
    I get 8.57 m/s using the equation F= mv~2/r - mg
    I think I need to use v = 2pi r/ T to figure out T. If I do this, I get T=5.49. I have no idea if this is right. If it is, then I'm not sure what 5.49 represents and how to turn it into rev/minute. The answer is supposed to be 11 rev/min.
  2. jcsd
  3. Jun 3, 2007 #2
    do you know what the relationship between [tex]V[/tex] and [tex]\omega[/tex] is?
  4. Jun 3, 2007 #3
    V = displacement / time elapsed,

    W = number of radians / second

    Is that right?

    Acceleration is A = change in velocity / change in time
    Angular accel. A = change in # of radians / change in time.

    This is about all I know. I'm not sure how they are related or how you get rev/min from here.

    Its probably staring me right in the face but I still can't see it
  5. Jun 3, 2007 #4
    You are on the right track with mv^2/r = mg, but that solves for the linear/tangential velocity (in m/s). Remember to convert that to angular velocity (rad/s) on your way to finding rpm.
  6. Jun 3, 2007 #5

    do you know the relationship between [tex]\omega[/tex] and rev/min?
  7. Jun 4, 2007 #6
    Is the relationship between W and rev/min: 0.1047 rad/s = 1 rev/min.
  8. Jun 4, 2007 #7
    yes, do you know why?
  9. Jun 4, 2007 #8
    No, no idea why. I just have that at the front of my textbook.
  10. Jun 4, 2007 #9
    [tex]2 * \pi[/tex]rad/s = 1rev/s right? cause [tex]2 * \pi[/tex] is the whole circle
    Last edited: Jun 4, 2007
  11. Jun 4, 2007 #10
    ok, that makes sense!


    I'm going to see if I can solve this problem now.
  12. Jun 4, 2007 #11
    nope, still lost!

    I think I'm stuck on where to start.

    I keep trying to figure out the velocity in m/s then convert to rev/min.

    Is there a better way to do it?
  13. Jun 6, 2007 #12
    i have the same problem...
  14. Jun 6, 2007 #13
    can any1 help?
  15. Jun 7, 2007 #14
    try to do a free body diagram showing all forces when the person is at the top of the ferris wheel
    then equate them
    one of them will have what u want in it
    and u are provided with what u don't know in the question

    let us know if u still cant get it
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