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Homework Help: Mass Question

  1. Sep 23, 2007 #1
    1. The problem statement, all variables and given/known data
    A lamp hangs vertically from a cord in a descending elevator that decelerates at 1.4 m/s2. (a) If the tension in the cord is 56 N, what is the lamp's mass? (b) What is the cord's tension when the elevator ascends with an upward acceleration of 1.4 m/s2?


    2. Relevant equations
    T-mg=ma


    3. The attempt at a solution
    I got part (b) to be 56 N and that was correct but then I used the formula... but I don't know how to solve for mass for part (a) when there are two masses in the equation above, I know T to be 56N and g to be 9.8m/s squared also a to be 1.4m/s squared... Can anyone help me out? I'm confused.
     
  2. jcsd
  3. Sep 23, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Just rearrange your initial equation:
    T-mg=ma

    Thus:
    T = mg + ma = m(g +a)

    You take it from here.
     
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