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<p><img src="https://s3.amazonaws.com/answer-board-image/b0272eaf-490b-4d3c-ad73-9b9016293034.jpg" alt="image from custom entry tool" />Can someone please just explain when adding net forces of an object, how I determine which forces are negative and which are positive? I originally thought it was just according to positive x and positive y axis. But on one problem with pulleys the net force of one side is T1(tension)-Mg and the net force of the other is 2Mg-T2. I just thought that the sign depended on if it was going toward the positive y direction or negative y direction. This is simple I know but I just want to be very clear</p>

Explanation / Answer

It is upto you. You can take upward force as +ve . Then downward has to be negative. Similarly force in right is +ve . Then force acting leftward as -ve. Normally , force that supports motion = +ve Force that opposes motion =-ve

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