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Homework Help: Calculation F=ma

  1. May 27, 2010 #1

    fcb

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

    Two masses of 40kg and 120 are connected by a light inextensible string

    Calculate the net acceleration of the system of masses
    magnitude and tension of the string
    29zones.png
    2. Relevant equations

    F=ma
    t=mg+ma
    t=mg-ma

    3. The attempt at a solution

    120x9.8 = 1176N
    40x9.8 = 392N

    1176 - 392 = 784N to the right
    784=(40+120) x a

    a=4.9


    BUT then i used the t=mg+ma and put in t = (40x9.8)+(40x4.9)
    and the answer then came out to be 588N

    where have i gone wrong

    one answer saids 784N the other saids 588N.
     
  2. jcsd
  3. May 27, 2010 #2
    Do a free body diagram on each mass. Tension is the same. You end up with 2 equations and 2 unknowns. The acceleration of the system is 4.9 m/s^2 and the tension in the string is 588 N. I don't see a problem with your analysis.
     
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