1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    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

    a_y = -g

    just to keep g a positive number.
  4. May 4, 2008 #3
    Ah ok I see. thanks
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook