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Homework Help: Easy Free Body Diagram Question

  1. Dec 2, 2015 #1
    1. The problem statement, all variables and given/known data
    Draw the free body diagram of the system. (I know that's not what the problem is, but that's what I need clarity on)

    2. Relevant equations

    3. The attempt at a solution
    So I drew the same thing, more or less, but for mine Ay went up. This was how a very similar problem was done - the difference being the whole system was on a plane instead of just the wheel. They say it goes down alongside the weight. So Instead of having my moments counteract each other they enhance each other!?


    Attached Files:

  2. jcsd
  3. Dec 2, 2015 #2


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    The weight of the horizontal bar tends to cause it to rotate counterclockwise about point B. Relative to the pin connecting the bar to the drum on the incline, there will be a vertical force which will resist the rotation of the bar, and there is also a horizontal force applied to the pin by the drum wanting to roll down the incline. Relative to the end of the bar at point A where the pin is located, this pair of forces will have their opposites applied to keep the bar and the wheel in equilibrium.
    What is 'it' referring to here?
  4. Dec 2, 2015 #3
    The force Ay on the left side of the bar... I believe you called it the relative to the end of the bar at point A where the pin is located.

    Here's how I visualized the Bar. I apologize for not calrifying in the first post... this would've helped:
    Ay goes up because the drum is pushing up on the bar, resisting the weight of the bar and the sum of the moments ought to be 0
  5. Dec 2, 2015 #4


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    The way you have Ax and Bx drawn, the bar would not be in equilibrium in the horizontal direction. Remember, the drum wants to roll down the incline; it's not stable where it is shown. The Ax force on your bar FBD is resisting the tendency of the drum to roll. Bx should be shown acting in the opposite direction of Ax for the bar.

    Ay for the drum is similarly keeping the bar from rotating about point B. Ay for the bar must be shown acting opposite to Ay from the drum. By for the bar also needs to be flipped so that the bar stays in equilibrium in the vertical direction.
  6. Dec 2, 2015 #5
    Oh yes, I see... that's very sensible when you put it that way. I also noticed that when they took the sum of the moments about B, they recieved a negative answer for Ay which implies that the arrow was incorrectly asumed.
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