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Math Help - Rearranging the formula

  1. #1
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    (Solved) Rearranging the formula

    Hello,
    Can anyone please show me how to rearrange the following formula making m the subject.

    J = mv - mu

    I am stuck before I start! You can't divide both sides by v, and you can't divide both sides by u (cuz that would make it worse!)

    anyways the answer should be J / v-u , which takes me to the other doubt.
    of you divide mv-mu by v-u (mv-mu/v-u), cancel out the v and u's giving m-m=0 (or is it m+m=2m?) either way it isn't m ...

    Thanks in advance
    Last edited by IBstudent; May 7th 2011 at 09:43 PM.
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  2. #2
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    Quote Originally Posted by IBstudent View Post
    Hello,
    Can anyone please show me how to rearrange the following formula making m the subject.

    J = mv - mu

    I am stuck before I start! You can't divide both sides by v, and you can't divide both sides by u (cuz that would make it worse!)

    anyways the answer should be J / v-u , which takes me to the other doubt.
    of you divide mv-mu by v-u (mv-mu/v-u), cancel out the v and u's giving m-m=0 (or is it m+m=2m?) either way it isn't m ...

    Thanks in advance
    One word: factor.
    J = mv - mu = m(v - u)

    Can you take it from here?

    -Dan

    Edit: \frac{mv - mu}{v - u} \neq m - m. When you cancel you have to cancel from both terms, not just one at a time. This is why you are getting a contradiction. A simpler example of this:

    According to your process
    \frac{6 \cdot 3 - 6 \cdot 2}{3 - 2} =  6 - 6 = 0

    but if you work out the numbers:
    \frac{6 \cdot 3 - 6 \cdot 2}{3 - 2} = \frac{18 - 12}{1} = 6

    You can see that the two are not equal.
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  3. #3
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    why didn't I think of that!
    really appreciate your help :-)
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