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Homework Help: Find mass using acceleration and tension

  1. Oct 8, 2011 #1
    1. The problem statement, all variables and given/known data
    A box is raised using a rope. If the acceleration of the box, is 5.2 m/s2 (upward) and the tension in the rope is 58 N, what is the mass of the box (in kg)?



    2. Relevant equations



    3. The attempt at a solution
    I thought T=ma would work, but yeah, it didn't.
     
  2. jcsd
  3. Oct 8, 2011 #2
    Methinks you're forgetting the gravitational force.
     
  4. Oct 8, 2011 #3
    The BEST FIRST STEP is DRAW A DIAGRAM with ALL the FORCES.
     
  5. Oct 9, 2011 #4
    so. My FBD looks like:
    T upwards with A
    And Weight downwards, so mg.
    so T+A = mg
    but, that didn't work either
     
  6. Oct 9, 2011 #5
    First, choose a proper set of coordinate axes(one +ve and other -ve) and then accordingly draw a free-body diagram.Finally,equate the NET force to ma .
    Please take note that A or a is the acceleration and NOT a force .
     
  7. Oct 9, 2011 #6
    I did draw that's what I just said.

    " so. My FBD looks like:
    T upwards with A
    And Weight downwards, so mg.
    so T+A = mg
    but, that didn't work either "
     
  8. Oct 9, 2011 #7
    Try using F(net) = ma (upwards)
     
  9. Oct 11, 2011 #8
    Yes, use F(net) = m*a(up) = m*5.2 (as given in the problem). In addition, F(net) = the sum of all the forces acting on the box, which, as you said, is the tension (given as 58N) and the weight.
     
  10. Oct 11, 2011 #9
    Note that Fnet is made up of two forces, the weight and the tension. But we want Fnet upwards. So one has to be careful what to put for Fnet because the weight is downwards but the tension is upwards. That is why THE FBD IS VERY IMPORTANT.
     
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