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Homework Help: Tension in two cables

  1. Feb 9, 2008 #1
    [SOLVED] Tension in two cables

    The problem statement, all variables and given/known data
    21.0 kg loudspeaker is suspended 2.40 m below the ceiling by two 3.90 m long cables that angle outward at equal angles.

    The attempt at a solution
    So my homework comes from the MasteryingPhysics online homework.

    What I first start out with is my free body diagram. I have 3 arrows, one pointing down for weight of the speakers by the earth (Wse) and two arrows pointing up diagonally (Ts1 and Ts2). Since the speaker is static you have Ts1 + Ts2 + Wse = 0.

    That was about as far as I could go. I then looked up information in my book but couldn't find much and so I resorted to the internet. I came across this page: http://science.palomar.edu/physics/Physics230/Tutor/4-31.html
    I used the equation they give T = mg/2sin(theta)

    I set a x-y axis starting with the speaker as the origin. I find the triangle that one string forms with the y-axis and the ceiling.

    I find the angle that corresponds with the picture on the page I found and plug in the information I have into the equation: T = (21*9.81)/(2sin(37.98))

    Putting this data into the answer box, I get the feedback: "Remember to use radians, not degrees, in arguments to trigonometric functions."

    I don't lose any attempts with the answer so I think I'm on the right track but I don't know what to do at this point.
    Last edited by a moderator: Apr 23, 2017
  2. jcsd
  3. Feb 9, 2008 #2
    It sounds to me like the angle is a 3-4-5 triangle. Don't use 2sin(37.98) in the denominator, it is at best an approximation.
    Use 2(3/5). When you are given those little reference triangles (dont know actual names), just remember your trig SOH CAH TOA. Since you are looking for the sin, which is S=O/H, that should give you 3/5. Thats all you need. Multiply it bye the magnitude 2.

    For your answer im getting 171.675.

    Mastering physics is a good HW site, but just be aware that they are occasional mistakes too.
    Good luck and I hope this helped.

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