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KE and PE

  1. Oct 10, 2011 #1
    1. The problem statement, all variables and given/known data
    Consider a frictionless roller coaster such as
    depicted below.
    The acceleration of gravity is 9.8 m/s2.
    Passenger cars start at point A with zero
    initial speed, accelerate as they go down to
    point B, swing around the circular vertical
    loop B ! C ! B of radius 30 m, then go
    on towards further adventures (not shown).
    When a car goes through the top of the loop
    (point C), the passengers feel weightless (for
    just a moment).
    What is the height hA of the starting
    point A above the loop’s bottom B?
    Answer in units of m

    2. Relevant equations
    KE = .5mv^2
    PE = mgh

    3. The attempt at a solution
    I know that the system is conservative energy since there is no friction and whatnot, so mgh(a) is the total energy. But what I don't get is, how does weightlessness come into this?
    If it's weightless, does that mean normal force doesn't exist?
  2. jcsd
  3. Oct 10, 2011 #2


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    Homework Helper

    did you forget ΣF = ma ? what about circular paths?
  4. Oct 10, 2011 #3
    then does gravity becomes the acceleration of centripetal force?
  5. Oct 10, 2011 #4


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    Homework Helper

    yes, to get the speed at the top ... Energy conservation then gets height at begining
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