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Homework Help: Dice Hanging from Mirroe

  1. Jan 28, 2007 #1
    1. The problem statement, all variables and given/known data
    A pair of fuzzy dice is hanging by a string from your rearview mirror. While you are accelerating from a stoplight to 29 m/s in
    5.7 s, what angle θ does the string make with the vertical?

    2. Relevant equations
    v_f=v_0+a*t
    x_f=x_0+v_0*t+0.5a*t^2
    v_f^2=v_0^2+2*a(x_f-x_0)
    x-x_0=[(v_0-v+v_f)/2]*2

    And just sum of forces=0
    F=ma

    Note: v_f^2 is final velocity squared

    3. The attempt at a solution
    I'm lost because it seems to me that the mass of the dice must be known to solve?
     
  2. jcsd
  3. Jan 28, 2007 #2
    This is an accelerated frame of reference. You know the direction of the acceleration that the frame has. There is a fictious force and the natural force (gravity), what is the vectorial sum of these two?
     
  4. Jan 28, 2007 #3
    Forget about ficticious forces. Draw a picture and a free body diagram. First, solve for acceleration. The tension force acts at an angle, which means a component of tension accelerates the dice. If you do things correctly, mass should cancel out of the equation
     
  5. Jan 28, 2007 #4
    A good idea would be to draw a force diagram outside and inside of the car =). To get a frame of reference for which way the dice are accelerating.

    They will be accelerating positively or negatively as the speed of the car =).

    Also, split this up into two vectors x and y. Once you start to do all the calculations, you'll find that the mass cancels out in the end =).
     
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