String and Pulley Problem

Aug 2011
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.

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


PHF Hall of Honor
Jun 2010
You can assume a direction, and it's okay.

Say you have acceleration a. If what happens is the opposite, then you will get a < 0 (meaning it moves in the other direction). Otherwise, if you were right, then you get a > 0.

Go ahead and work it out! :)
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