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Homework Help: Angle between two ropes

  1. May 13, 2010 #1
    Allie, Beth, and Cathy are having a 3 way tug of war. Allie is pulling with a force of 30N, Beth with 50N, and Cathy with 70N. If the system is in equilibrium, what is the angle between Allie's rope and Beth's rope.

    I used the cosine law, and

    [tex]cos\theta=\frac{{30}^2+{50}^2-{70}^2}{2\cdot 30\cdot 50}[/tex]
    [tex]\theta = 120[/tex]

    But the answer is 60, because 180-120=60. Why?
     
  2. jcsd
  3. May 14, 2010 #2
    The angle you have calculated in the force triangle (shown by the arrow) is not the angle θ between the two ropes A and B.
    forces.png
     
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