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Apparent weight - Rotor ride

  1. Dec 1, 2015 #1
    1. The problem statement, all variables and given/known data
    A 30-kg child is on a rotor ride at a carnival. The radius of the rotor is 3 m and it completes 0.4 revolutions in one second. The coefficient of static friction is 0.6. What is the magnitude of the child's apparent weight?

    rotor1.png
    2. Relevant equations
    ΣFx : FN = mv2/r
    ΣFy : Ff = Fg

    3. The attempt at a solution
    First I need to find v:
    0.4 revolutions per second is equivalent to one revolution per 2.5 s, so T = 2.5 s.
    v = 2πr / T => v = 7.54 m/s.

    Since the apparent weight is the magnitude of the resultant force exerted on a body by a supporting surface, the way I see it FN is the only force that fits this description so I figured
    apparent weight = FN = mv2/r = 568.5 N.

    However the solution given by the author says that
    apparent weight = (FN2 + Fg2)1/2 = 640 N.

    I've been trying to understand why mg plays a role in the child's apparent weight but I don't see how in this case
     
  2. jcsd
  3. Dec 1, 2015 #2

    TSny

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    Hello and welcome to PF!

    Are you sure that FN is the only force exerted on the person by the wall? Consider the forces shown in your figure.
     
  4. Dec 1, 2015 #3
    Thanks!

    I guess the friction applies in this case but for some reason because of its direct relationship with the normal force I didn't consider it. It makes a lot of sense though now that I think about it
     
  5. Dec 1, 2015 #4

    TSny

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    OK, good.
     
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