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Tension and Newton's second law

  1. Oct 12, 2016 #1
    1. The problem statement, all variables and given/known data
    A 7kg mass is hung from a 1.5 m long massless string. It is released from rest with the string displaced by
    an angle of 18 degrees from vertical.
    a) Make a free body diagram of the forces on the mass just after it is released.
    b) Find the tension in the string.
    c) Find the initial acceleration of the mass.

    2. Relevant equations
    F = ma

    3. The attempt at a solution

    b) Fy: Tcos18 - 68.6 = ma

    Does ay = 0? And if it does then why exactly? Is it just cause they want the tension at that point so the object is not assumed to be moving?

    However there will be an ax value right?

  2. jcsd
  3. Oct 12, 2016 #2


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    In which direction does the tension act just after the mass is released? (What is the string actually accomplishing in this system?)
  4. Oct 12, 2016 #3

    Attached Files:

  5. Oct 12, 2016 #4
    That is the diagram I made. Tension and Fg are in opposite directions which is how I got Tcos18 - 68.6 = ma
  6. Oct 12, 2016 #5


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    Science Advisor

    According to your diagram, Tension and Fg are not exactly in opposite directions. Just kind of.
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