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Homework Help: Person sitting on scale on roller coaster

  1. Nov 11, 2012 #1
    1. The problem statement, all variables and given/known data

    A 120 pound person is sitting on a scale during a roller coaster ride. As the roller coaster goes over the top of a curve whose radius is 30 feet, the scale indicates a force of 25 pounds. The average speed of the roller coaster doesn't change while over this curve.

    A) What forces are acting on the person?
    B) Specify the magnitude and direction of each of the forces
    C) Draw a free body diagram of the person
    D) Describe the acceleration of the person
    E) How fast was the roller coaster moving at that time?

    2. Relevant equations

    I know that a=mv(squared)/r : This equation goes into the right side of the ƩF=ma.
    I know how to draw a free body diagram, start with the object at the center and draw the forces outward.

    3. The attempt at a solution

    A) I believed the forces to be: Fg=weight of person; Fn=force perpendicular to interface; Ff=mv(squared)/r
    B) Fg=120pd, down; Fn=up, don't know magnitude; Ff=(120)v(squared) and I believe this is pointing inward, but I'm kind of confused by that.
    C) This I am able to draw if I know the correct direction of the forces.
    D) I have no idea how to get the acceleration, but I do have the equation. a=v(squared/r). Problem is, I only have the radius so I don't know how to go about solving for acceleration.
    E) This also means I don't understand where to start to figure out the velocity of the roller coaster.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

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  3. Nov 11, 2012 #2


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    welcome to pf!

    hi cdornz! welcome to pf! :smile:

    (try using the X2 button just above the Reply box :wink:)
    what is "Ff" supposed to be an abbreviation for? :confused:

    mv2/r is not a force, it's m times acceleration, and as you say, it goes on the RHS of ∑F = ma
    yes, you do know the magnitude of Fn: read the question!

    (where does Fn come from?)
    i don't understand :confused: … r is the radius
  4. Nov 11, 2012 #3
    Re: welcome to pf!

    Ff is supposed to represent force of friction, which I thought there would be since this is technically steel on steel.

    As for the magnitude of Fn, would it be 145 pounds? I would assume that is the amount, because that's the amount of force pushing down..I wasn't sure if Fg and Fn would be equal or not.

    As for acceleration, radius isn't needed, but I wasn't sure with the correct equation if I could solve for it.
  5. Nov 11, 2012 #4


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    it's rolling … this is a rollercoaster

    there's no friction (with the ground or the rail) when something rolls!

    (and anyway why would it equal mv2/r ?)
    why would it be more than the weight? :confused:

    anyway, what object is this reaction force coming from?
    i honestly have no idea what you're talking about :redface:

    what equation do you propose to use for the acceleration?​
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