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Homework Help: Calculating Tension when you swing a ball around a piece of string

  1. Nov 5, 2014 #1
    1. The problem statement, all variables and given/known data
    You swing a ball around a piece of string(vertically). M = 0.150kg r= 1.10m
    Calculate the Tention at the bottom of the arc assuming velocity is twice as fast.

    2. Relevant equations
    When I do the problem i'm not sure how to set up the NetForce = ma
    How do which force is negative and which is positive? I set tension at the bottom to - making the equation to be
    mg - Ft = m(v^2/r)
    but i dont get right answer

    3. The attempt at a solution
    mg - Ft = m(v^2/r)
    ft = -4 N
    but the answer is 7.35 N
  2. jcsd
  3. Nov 5, 2014 #2
    Actually I think i just figured it out. The reason it needs to be positive and gravity has to be negative because at the bottom of the path T is pointing inwards because this is Ar. Is this the correct logic? I appreciate it.
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