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Homework Help: A pendulum on a jet plane

  1. Sep 17, 2012 #1
    1. The problem statement, all variables and given/known data
    A pendulum has a length L = 1.47 m. It hangs straight down in a jet plane at rest, as shown by the dotted line in the figure. Then the jet accelerates uniformly, and during that time, the pendulum moves to the equilibrium position shown by the solid line, with D = 0.39 m. What is the acceleration of the plane?

    2.Relevant equations f=ma

    3. The attempt at a solution
    when i drew this out i thought that we needed to have theta because its swinging at an angle but we dont have theta and iam stuck bc iam not sure if iam supposed to use a constant acceleration formula...
  2. jcsd
  3. Sep 17, 2012 #2
    It would help immensely if we had that figure the question is referring to.
  4. Sep 17, 2012 #3
    2 Things first.
    1. Break down your components into x,y,z.
    2. What do we know in this situation (or can assume)? It's static! There is no movement!
    I'll start you off:
    dp_x/dt = 0 = F_t*cosx - F (= ma)
    dp_y/dt = 0 = F_t*sinx - mg
    dp_z/dt = 0 (I don't think anything is happening in the z direction..)
    Basically since F = dp/dt = 0 since there is nothing changing right? The momentum of the pendulum is still (or static) therefore the net force must be zero. (F=ma is actually wrong, learn more about it later though - Einstein)
    Now solve for ma
    Last edited: Sep 17, 2012
  5. Sep 17, 2012 #4
    there is no pic for this problem.
    post3. i dont understand what you r writing. i thought that the pendulum is swinging due to the acceleration of the plane which is what iam trying to find...
  6. Sep 17, 2012 #5
    the problem is basically asking this: the pendulum starts at rest inside of a jet plane, so it is hanging straight down.

    The jet plane then accelerates, so that the pendulum is pointing at some angle backwards. The acceleration is constant and uniform. What acceleration is needed to cause the pendulum to stay at this position?

    So just think about the forces acting on the pendulum.
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