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Need help with Newton's Law problem.

  1. Sep 28, 2009 #1
    1. The problem statement, all variables and given/known data

    Two masses are joined together by a massless, inextensible string. A vertical force of 30 N applied to the upper mass gives the system a constant upward acceleration of 3.2 m/s^2.

    If the tension in the connecting string is 18 N, what is the smaller of two masses?

    If the tension in the connecting string is 18 N, what is the bigger of two masses?

    F= 30 T=18 m=1.4 a=3.2 g=9.8

    2. Relevant equations

    F(net) = m*a

    3. The attempt at a solution

    The lower block can be solved for by:
    T/(g+a) = M which is 1.4kg

    but I can't get the value of the upper block(which I know should be smaller then 1.4)

    M=(F+T-ma-mg)/(a+g) F=30 T=18 m=1.4 a=3.2 g=9.8
    when I do this M= 2.3 but it should be smaller then 1.4 because I know 1.4 is the larger block.
  2. jcsd
  3. Sep 28, 2009 #2


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    Call the bottom mass m2. You know its acceleration is 3.2 m/s2. You also know that the tension acting on it is 18 N. Can you set up a Newton's Second Law equation for that mass alone in which m2 is the only unknown?
  4. Sep 28, 2009 #3

    Doc Al

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    Staff: Mentor

    You messed up the forces in that equation. (And what's "ma" doing in there?) Just use Newton's 2nd law directly, rather than plug into a derived equation that may or may not apply. It's much simpler and you'll know it's right.

    What's the net force on the upper mass?
  5. Sep 28, 2009 #4
    Isn't the net force 48 because both the 30 N and the 18 N of tension are moving in an upward direction.

    48 = M(3.2-9.8)?

    *edit: never mind I found the solution.. Thanks so Much!"
  6. Sep 28, 2009 #5

    Doc Al

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    Staff: Mentor

    (1) The 30 N and 18 N forces act in different directions.
    (2) The weight of the mass contributes to the net force.
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