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Homework Help: One Knot Problem - Find two unknown masses

  1. Oct 1, 2009 #1
    1. The problem statement, all variables and given/known data

    See the attachment.
    We're asked to find the masses of M1 and M2. I'm wondering how this is possible, 'cause there's two variables and only one known? Do I have to use sine/cosine law?

    m3 = 0.1998 kg
    T3 = (0.1998 kg)(-9.80 m/s^2) = -1.96 N

    Vertical angle A (between T3 and T1) = 38.0 degrees
    Vertical angle B (between T3 and T2) = 27.0 degrees

    2. Relevant equations

    F = ma
    F = ma sin (Θ)

    3. The attempt at a solution

    I tried to solve with algebra:

    m1 g sin (A) + m2 g sin (B) = m3 g

    Gravity cancels.

    m1 sin (A) + m2 sin (B) = m3 g

    ...And that's about as far as I got :(

    Any help/hints would be appreciated muchly.. the Physics head decided we should only get two days to do our labs.. it's due tomorrow.

    Attached Files:

  2. jcsd
  3. Oct 3, 2009 #2


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    Homework Helper
    Gold Member

    Hint: Force is a vector...it will have both horizontal and vertical components, and both must be zero for the mass m3 not to move....
  4. Oct 3, 2009 #3
    Yeah, I got it now. Had to:

    Solve for m1 in the equation for the sum of all the x components and the equation for the sum of all the y components.

    Then make them equal to eachother, solve definitively for m2, then substitute to solve for m1.

    Thanks for the help!
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