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String and Pulley Problem

  1. Oct 15, 2011 #1
    1. The problem statement, all variables and given/known data

    The masses of blocks A,B, and C are 4kg, 10kg and 2kg respectively. Knowing that P=0 and neglecting the masses of the pulleys and the effects of friction, determine a) the acceleration of each block and the tension in the chord.

    2. Relevant equations


    3. The attempt at a solution
    I know that we have to draw FBD's for each block and then figure out the equations of each Fnet. But first, how do you know the direction that this system will move? Because for example, if you know that block A is moving down, we can assume down is positive and its equation will be
    ma=mg-2T (because we denote down as positive)? Also, for block B, will the tension be 4T, since it's connected to four strings?
    I am confused about how system works.


    Physics Problem Set.jpg
  2. jcsd
  3. Oct 15, 2011 #2


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    Gold Member

    Just pick directions that you think the masses will accelerate and then write out you equations accordingly. If you picked the wrong direction the acceleration will have a minus sign in the end.


    Attached Files:

  4. Oct 16, 2011 #3
    Oh, okay thanks for the diagram! What is y and V though?
  5. Oct 16, 2011 #4


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    Gold Member

    y_2, y_4, and y_10 label the distances the masses move and V_2, V_4, and V_10 are the velocities of those masses. The directions chosen are my guess as to which way the masses will move.

    Good luck!
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