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Homework Help: Deriving an Equation

  1. Sep 21, 2008 #1
    I am supposed to derive this equation: g=aave(M + m / m)

    It gives me two pairs of equations:
    T1-Fr = Ma1
    mg-T1= ma1


    T2 + Fr = Ma2
    mg - T2 = ma2

    It also says, "hint: for the first pair of equations, eliminate the unknown tension. Do the same for the second pair of equations. In this new pair of equations, eliminate the unknown force of kinetic friction Fr.

    So I tried just canceling out the T in the first set of equations, leaving me with
    -Fr = Ma1 and mg = ma1, but that doesn't seem right. I am mostly just confused on how to "derive an equation". I'm just looking for a little direction--do I set them equal to each other? I'm just confused on what it's asking, I've never done a problem like this before and have been trying to understand it forever! Any help would be greatly appreciated. Thanks!!
  2. jcsd
  3. Sep 22, 2008 #2


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    Gold Member

    When it says to cancel the T's out, it doesnt mean to literally pretend they never existed. To eliminate the T's from the first two equations, re-write them so that the T's are isolated in both equations.

    Now it should be obvious that if A=C and B=C, then A=B. Use the exact same idea with the equations, and see if you can work from there.
  4. Sep 22, 2008 #3
    Thanks that helped, I think I got it now!
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