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Can M3 move upwards and M2 move to the right? in the solutions they assume this, but I find it impossible, it doesn't make sense physically. I get the answer wrong if I assume M3 moving down and M2 moving right in the next question, which is the one that makes sense.

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  • $\begingroup$ If the string is constant length, $M_3$ must move down as $M_2$ moves right. Be careful about the signs of your accelerations, and don't ignore the reaction force on the pulley. $\endgroup$ – ArbiterKC Jun 28 '14 at 20:49
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Well, Let's assume F = 0 -> M3 goes down and M2 to the right.

Now assume F > M3g -> M2 goes to the left and M3 goes up.

So that's 2 cases, and they can be solved in 1 set of equations. Those equations assume M3 and M2 are free to move in whatever direction they please... The correct paths will be given by doing the math properly.

I suppose that's why they assume this. Assume the string is always at tension and doing the math will give you the right directions.

Just keep a sharp eye for signs and directions.

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