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Homework Help: The net force?

  1. Nov 16, 2007 #1
    help, please :)

    1. The problem statement, all variables and given/known data
    Two masses are suspended by a string placed over a pulley supported by the ceiling (m1 = 2.20 kg and m2 = 7.70 kg) Consider up to be positive.
    What is the acceleration on the 6.20 kg mass? The net force?
    What is the acceleration on the 7.70 kg mass? The net force?

    2. Relevant equations

    m2g / (m1+m2) (is that the correct one?)

    F = MA

    3. The attempt at a solution

    For the 6.20 kg mass:

    7.70(9.81) / (7.70 + 6.20) = 75.537 / 13.9 = 5.43

    6.20 * 5.43 = 33.7 N

    Am I anywhere close??
  2. jcsd
  3. Nov 16, 2007 #2


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  4. Nov 16, 2007 #3
    YES. wow that is helpful! thank you.
  5. Nov 16, 2007 #4
    Ok, now my problem is, there is no mention of tension. So how do I solve for the individual net forces if I don't know acceleration or tension?
  6. Nov 17, 2007 #5


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    Refer to http://hyperphysics.phy-astr.gsu.edu/hbase/atwd.html#c2

    The acceleration is numerically the same for the two masses. The tension acts upward for either mass.

    One can find T in terms of the lighter mass, and one knows a in terms of the masses and g, the acceleration of gravity.
  7. Nov 17, 2007 #6
    wonderful. thank you!
  8. Nov 17, 2007 #7
    After solving, I found the acceleration of both to be 1.06

    The individual net forces are 6.57 for the 6.20 mass, and 8.16 for the 7.70 mass. Do you believe this is correct?
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