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Homework Help: Question Concerning F=ma and FBDs.

  1. Oct 17, 2005 #1
    Ok, I know that, using a FBD, one is susposed to use [tex] \sum F = m a [/tex]
    My question is what order do you put the forces that [tex] \sum F [/tex] equal
    An example might help me explain my question.
    Lets take a block of wood that is supported by a string. The only forces acting on it are [tex] mg [/tex] and [tex] T [/tex]
    [tex] \sum F = ma [/tex]
    [tex] T - mg = ma [/tex]
    [tex] mg - T = ma [/tex].
    I know if [tex] a = 0 [/tex] it does not matter, but how does one decide what order to put th forces in when there is acleration?
    Last edited: Oct 17, 2005
  2. jcsd
  3. Oct 17, 2005 #2
    The [tex] \sigma [/tex] should be a capital sigma.

    I fixed it.
    Last edited: Oct 17, 2005
  4. Oct 17, 2005 #3


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    Homework Helper

    It depends on how you choose the positive and negative directions.
    T - mg = ma1, you've chosen the positive direction upwards.
    mg - T = ma2, you've chosen the positive direction downwards.

    T - mg = ma1 = -(mg - T) = -ma2

    In the case that mg is greater than T, the acceleration's negative when you've chosed the positive direction upwards, and positive when you've chosen the positive direction downwards. The magnitude does not change.
  5. Oct 17, 2005 #4
    So on an Atwood machine, I should chose it so that both masses are the same way written (both T - mg, or vice versa), or so that the greater force is first.
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