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Homework Help: Help with deriving an Equation from 4 others

  1. Oct 25, 2009 #1
    1. The problem statement, all variables and given/known data

    Our teacher assigned us this problem of trying to derive and equation from 4 simpler equations and I just can't seem to get the correct answer so I need some help.

    a= M1g / (1/2)M2+M1

    a= acceleration
    M1= Mass 1
    M2= Mass 2
    g= gravity
    R= radius
    T= tension
    I= moment of inertia
    [tex]\alpha[/tex]= angular acceleration

    2. Relevant equations
    These are the four equations at which I am supposed to derive the equation above from

    a) RT = I[tex]\alpha[/tex]
    b)M1g-T = M1a
    c)a = [tex]\alpha[/tex]R
    d) I = (1/2)M2R2

    3. The attempt at a solution

    I am very close to the answer but no cigar


    a= (RT/I)*R

    a= [ RTR / (1/2)M2R2]

    a= T / (1/2)M2

    a= -(M1a) + M1g / (1/2) M2

    a= (M1g) / ((1/2)M2 + M1a

    As you can see I can get all of it correct except for the "a" on the bottom of the equation is wrong. Does anyone have any idea on what to do here? Thanks in advance.
  2. jcsd
  3. Oct 25, 2009 #2
    How did you get the last step? if you want 'a' just group them together.
  4. Oct 25, 2009 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    It looks good up to here, but you really should use parentheses to group the terms properly:

    a= -(M1a + M1g) / ((1/2) M2)

    I suggest multiplying on both sides by (1/2)M2, so that you won't have a complicated fraction to deal with. Then it will be easier to use semc's suggestion of grouping the a's together.
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