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Homework Help: Find Forces Using Tension

  1. Oct 16, 2015 #1
    1. The problem statement, all variables and given/known data
    In the figure below, the objects are attached to spring balances calibrated in newtons. Give the readings of the balances in each case, assuming that the strings are massless and the incline is frictionless.


    I need help with (d). If the picture's too grainy to read off the values, the two blocks weight 10 kg each, the angle is 30 degrees, and the sides are 5.2 m (right above the (d)) and 3 m.

    2. Relevant equations

    F = ma, with Σ F = 0

    3. The attempt at a solution

    So, I know the reading off the balance will be equivalent to the tension in the string. I've tried doing this problem two different ways: first, I found the total Weight force (98.1 N) and used the sin to find the hypotenuse, which was 196.2 N. Then, when that was wrong, I thought maybe I should use gravity, split it into components, and then multiply whatever the hypotenuse of that right triangle was by the mass. I got 196.2 N again. What am I doing wrong? I've drawn a free body diagram and everything, but I don't know what I'm doing wrong here. I have a suspicion I'm not drawing the forces correctly, but I'm not sure. Thanks in advance for any help!
  2. jcsd
  3. Oct 17, 2015 #2
    How can the spring balance give a value greater than the weight of the block?

    Maybe show your FBD and your calcs.
  4. Oct 17, 2015 #3
    You determined the component of the weight parallel to the incline incorrectly.

  5. Oct 17, 2015 #4
    I think I figured it out. I was drawing my triangle incorrectly (the amount of silly mistakes I make when doing physics problems...). Thanks for the sanity check, paisiello2, and for hinting at what was probably wrong, chestermiller. I appreciate it!
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