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Finding tensions from incline

  1. Dec 14, 2014 #1
    Blocks 1 and 2 of masses ml and m2, respectively, are connected by a light string, as shown above. These blocks are further connected to a block of mass M by another light string that passes over a pulley of negligible mass and friction. Blocks l and 2 move with a constant velocity v down the inclined plane, which makes an angle q with the horizontal. The kinetic frictional force on block 1 is f and that on block 2 is 2f.

    I know that the coefficient of friction would be Ff/m1(-9.8)cos(angle).
    How would I find the value of the suspended mass M that allows block 1 & 2 to move with constant velocity?
     

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  3. Dec 14, 2014 #2

    haruspex

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    You won't need to consider the coefficient of friction to answer this part.
    Do you understand how to develop free body diagrams when there are multiple interacting bodies? You create unknowns for all of the forces of interaction (the tensions here), draw a free body diagram for each body, and write down the dynamical equations for each body. You also need to make use of kinematic facts, like the fact that some string length does not change. This allows you to deduce that some velocities or accelerations are the same. Where a body has no mass (the pulley here) you know the sum of the forces and sum of torques are zero.
     
  4. Dec 14, 2014 #3
    So to find the suspended mass M would I create a free body diagram showing all of the forces acting on it then find what equation to find it? Would the Equation involve using sin or cos with the angle?
     
  5. Dec 14, 2014 #4

    haruspex

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    That alone won't tell you the mass. You need a free body diagram for each object in the system, and equations obtained from them. When the number of equations matches the number of unknowns, you probably have enough information to solve it.
    Since the pulley is massless and frictionless, we can avoid an FBD for that; you just have to figure out how the tension on one side of it relates to the tension on the other side. So three FBDs should do it.
    In general, you can get three equations per FBD: two for linear forces along some chosen co-ordinates (often, horizontal and vertical) and one for torque. In the present case there are no torques. So you could get up to 6 equations, but you won't need them all.

    What equation do you know relating forces and accelerations in general?
     
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