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Homework Help: Frictional force in a free-body diagram

  1. Feb 9, 2013 #1
    1. The problem statement, all variables and given/known data
    I am having trouble understanding the direction of a friction force in a sample problem from Engineering Mechanics, Meriam & Kraige. I will attach a picture for no confusion.


    3. The attempt at a solution

    Consider only the top free-body diagram. Why is the friction force in that direction?

    "The friction forces are assigned in the directions to oppose the relative motion which would occur if no friction were present." So where would the object move if there was no friction? Well, it would not since it is attached to a wire. If not attached, it would(or tendency) to move downward and therefore an frictional force in the opposite direction drawn. So, from my own conclusion I would have chosen:

    1) Upward force or 2) No friction force at all since it would not move if there was no friction.

    Why am I wrong?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 9, 2013 #2
    The object held by the wire would not move. But the object just below would.
  4. Feb 9, 2013 #3
    Let's say that the 50kg-block would move. Then I assume it would move downward and therefore there is a friction force F_1 in the upward direction, opposite the motion, exactly as in the SECOND free-body diagram. But that doesn't explain why the friction force in the TOP f-b diagram is in the downward direction. Right?
  5. Feb 9, 2013 #4
    Apply Newton's third law.
  6. Feb 9, 2013 #5
    Alright, I think I see it now. Thanks voko.
  7. Feb 9, 2013 #6


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    Homework Helper


    The friction opposes relative motion between the surfaces in contact. If the 50 kg block moved it would leave the top block behind without friction. So the force of friction between the blocks tends to move the top block together with the 50 kg one downward.

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