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Classic mechanics problem:two masses with a wedge

  1. Sep 27, 2014 #1
    1. The problem statement, all variables and given/known data
    Given that two masses are connected by a light string over a pulley.They are initial at rest.Suppose all the surfaces are smooth.Find the resultant acceleration of the system

    2. Relevant equations
    3. The attempt at a solution
    Suppose I let the resultant a be A,and the acceleration of each mass along the wedge be a
    First for mass m
    for mass m'
    But then i get stuck because there are 5 unknown but there are only 4 equations
    Did I miss anything?

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  3. Sep 27, 2014 #2


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    First off, it is not completely clear what you mean by "resultant", are you implying that it is the tension+gravity? In that case you are essentially trying to take them into account twice. Second, you do not need to consider the force equations orthogonal to the directions of motion as the normal force will settle this to give no direction. You will then have two equations (one from the free body diagram for each block) and two unknowns (the tension in the string and the acceleration). What other unknowns are you thinking of? The angles x and x'? Those are parameters of the problem and your final solution will depend on them. (However, note that there is a simple relationship between them due to the geometry of the problem.)
  4. Sep 27, 2014 #3
    I think what the resultant acceleration means is that A,which is acceleration of the whole system moving left or right
    I still get 3 unknowns,a,A and T,in two equations
  5. Sep 27, 2014 #4


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    You are saying that the wedge is also allowed to move? This is not clear to me from the problem statement and of course makes the problem significantly harder. In that case you will have to consider several additional things. Would the "resultant acceleration" be the acceleration of the wedge or the acceleration of the centre of mass of the entire system?
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