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Car door shutting at a certain acceleration

  1. Feb 21, 2010 #1
    1. The problem statement, all variables and given/known data
    A car has its door open at 90 degrees. The door is considered a uniform square sheet of steel of side .8 m and mass 20kg. The hinges on the door are frictionless. At time t = 0 the car accelerates with constant acceleration a = 3 m/sec^2. How long does it take the door to close?

    2. Relevant equations
    Torque = Mass * radius * acceleration
    acceleration = radius * alpha
    Torque = Mass * radius^2 * alpha

    3. The attempt at a solution
    T = 20 (Mass) * (.8)(Radius) * 3 (Acceleration)
    T = 48
    alpha = 48(Torque) / (.8^2)(Radius) * (20)(Mass) = 3.75
    Now that I have angular acceleration, I need only to integrate twice to find the time.
    omega = 3.75t
    theta = 3.75t^2 / 2 theta = 90 degrees or pi/2 in radians so,
    pi/2 = 3.75t^2 / 2 pi/3.75 = t^2 t = sqr root (pi/3.75)

    t = .92 seconds

    Hopefully these are the correct formulas, but I could use someone to check if these are right. Thanks in advance.
    Last edited: Feb 22, 2010
  2. jcsd
  3. Feb 22, 2010 #2


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    Science Advisor
    Homework Helper

    Hi Mastablade! :smile:

    Sorry, I don't understand any of this. :redface:

    What is the torque ?

    Why are you not using moment of inertia ?

    What physical principle are you using ?

    Go back to your book, and read the chapter again. :smile:
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