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Homework Help: Roller coaster

  1. Feb 28, 2009 #1
    1. The problem statement, all variables and given/known data
    The normal force equals the magnitude of the gravitational force as a roller coaster car crosses the top of a 60m diameter loop-the-loop.

    2. Relevant equations
    What is the car's speed at the top?

    3. The attempt at a solution
    hello, this problem doesn't give much information
    would normal force be pointing down?

    so Fnet= ma=mv^2/r but its doesnt give those variables

    can anyone help?
    thanks :)
  2. jcsd
  3. Feb 28, 2009 #2


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    Don't you have all you need?

    Draw a diagram.

    m*g down and mv2/r up

    But it says the Normal force is still m*g

    If the net force must be m*g and you have m*g acting down against the centripetal force, what must the centripetal force be?
  4. Feb 28, 2009 #3
    Is the net force in the y direction zero?

    does the centripetal force = 2mg since normal and gravity both point downward?

    im lost since this coaster is upside down
  5. Feb 28, 2009 #4


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    Not lost at all.

    The centripetal force is 2*m*g.
  6. Feb 28, 2009 #5
    so would net force in the y direction = zero?

    if it is zero, i dont think it would be solvable because we dont know the mass

    wouldnt it be 0=mv2/r
  7. Feb 28, 2009 #6
    ok, i got the answer but i used the net force=2mg
    i still dont understand why this is so
  8. Feb 28, 2009 #7


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    FNormal = ∑ F = Fcentrip - Fgravity

    m*g = m*v2/r - m*g

    2*m*g = m*v2/r
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