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Newton's Laws - Positive & negative direction

  1. Jul 8, 2017 #1
    While solving homework problems, I start by defining a positive-y direction and a positive-x direction. Let's say up and to the right. Until now, I've applied this same rule to all objects in each problem, no matter the direction the object moves. But it seems I have not understood this fully.
    While solving the problem for the attached picture, I had to define a positive direction up for box A, and a positive direction down for box C to get the right answer. Is the positive direction decided by which direction the object moves in constant acceleration problems like this?

  2. jcsd
  3. Jul 8, 2017 #2
    The reason you have to do this is because you have chosen to keep a always positive. A consequence of this is that the force of gravity is positive for block A and negative for block C to make F=ma come out right.
    Thats not how I would have done it. I would have chosen up is positive always, and the acceleration for A would be a, and the acceleration for block C would be -a.
  4. Jul 8, 2017 #3
    I agree! I recall doing it this way before, had forgotten all about it. Thank you :-)
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