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Homework Help: 2 dimensional equilibrium

  1. Oct 18, 2011 #1
    1. The problem statement, all variables and given/known data
    The stretchy silk of a certain species of spider has a force constant of 1.50 . The spider, whose mass is 15mg, has attached herself to a branch as shown in the figure.
    a)Calculate the tension in each of the three strands of silk.
    b) Calculate the distance each strand is stretched beyond its normal length.

    2. Relevant equations
    Worked with the equations: Tc=Tacos(30) + Tbcos(40) for vertical components where Tc=mg=147 mN. And for the horizontal Tasin(30) = Tbsin(40). Found the ratio and plugged back into the equations to find Tb and then Ta.

    3. The attempt at a solution
    Using the relevant equations, I derived Ta = Tb x 1.286 and ultimately got answers:
    Ta=100mN, Tb=78.2mN, and Tc=147mN.
  2. jcsd
  3. Oct 19, 2011 #2
    re tensions: chech 147mN

    re last part: use force = force constant x extension
  4. Oct 19, 2011 #3
    Does that say check* 147 mN, in that it is an incorrect magnitude for Tc?
  5. Oct 19, 2011 #4
    A mass of 15mg = 15/(1000) = 0.015grams = 0.015/(1000) = 0.000015kg and now multiply by 9.81 to give weight in N to give 0.000147N = 0.147mN
  6. Oct 19, 2011 #5
    so it should come to .100mN, .0782mN, .147mN for Ta, Tb, and Tc respectively?
  7. Oct 19, 2011 #6
    the answer needs to be in 2 sig figs. I put in .100, .078, and .15. A message came up and said "Term 2: Not quite. Check through your calculations; you may have made a rounding error or used the wrong number of significant figures."

    I cannot find this error. Kept all numbers in my calculator.

    Same response for the second part, in which I submitted .067, .052, and .098
  8. Oct 19, 2011 #7
    I think that the angles were interpreted incorrectly.

    The angle beteen tension A and tension B looks to me to be more than 90 degrees. So I think that the angles are 30deg and 70deg, together making up 100deg.
  9. Oct 19, 2011 #8
    worked out. Thanks a lot
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