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Homework Help: Simpe Force Question

  1. May 4, 2008 #1
    [SOLVED] Simpe Force Question

    1. The problem statement, all variables and given/known data
    A 2.00 kg object placed on a frictionless, horizontal table is connected to a string that passes over a pulley and then is fastened to a hanging 7.00 kg object, as in Figure P5.24. Find the magnitude of the acceleration of the two objects and the tension in the string.
    3. The attempt at a solution
    I have a specific question about this problem that I came upon at the end. Here is how I solved it.

    [tex]T = m_1a[/tex]
    [tex]-T + m_2g = m_2a[/tex]
    So two equations to uknowns, and here is what I got.
    [tex]-m_1a + m_2g = m_2a[/tex]
    [tex]a = \frac{m_2g}{(m_1+m_2)}[/tex]

    So solving for a I got -7.62. The thing I have a question about is the negative on the acceleration. At the beginning of the problem I defined rightward and downward direction as positive and upward direction as negative. So if acceleration is a negative value does that not mean it is traveling up, since I defined up as negative? I know that it doesn't not, but I am confused it can be this way.
     
  2. jcsd
  3. May 4, 2008 #2

    alphysicist

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    Homework Helper

    Hi Sheneron,

    How did you get a negative answer? All the values (masses and g) are positive numbers.

    Did you use g=-9.8? The value of g is a magnitude and therefore always a positive number; if we were talking about an object in free fall we would say that its acceleration is

    [tex]
    a_y = -g
    [/tex]

    just to keep g a positive number.
     
  4. May 4, 2008 #3
    Ah ok I see. thanks
     
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