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Math Help - Rearrange formula

  1. #1
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    Post Rearrange formula

    Not sure if i have posted this in the correct Forum so sorry if it is incorrect But i am desperate to get this answered.

    I have an existing formula that i use in work to find out the OD of a coil from the weight, width and ID of the coil but i need to change it so i can find the weight from the ID, OD and width of the coil. The Formula i use is:

    OD=(SquareRoute((Coil Weight*1000/(24.66*Coil Width))+(Coil ID/2000*Coil ID/2000)))*2000

    Please help its been driving me mad for the past 5 hours.
    Last edited by CaptainBlack; April 8th 2011 at 07:41 PM. Reason: To give thread a descriptive title
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  2. #2
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    Quote Originally Posted by chriscmartin View Post
    Not sure if i have posted this in the correct Forum so sorry if it is incorrect But i am desperate to get this answered.

    I have an existing formula that i use in work to find out the OD of a coil from the weight, width and ID of the coil but i need to change it so i can find the weight from the ID, OD and width of the coil. The Formula i use is:

    OD=2000\left(\sqrt{\dfrac{1000W}{24.66c_w}}+\dfrac  {ID}{2000} \cdot \dfrac{ID}{2000}\right)

    Please help its been driving me mad for the past 5 hours.
    I've edited the quote into it's latex form to make it easier to read. However, I may have made a mistake, is that the correct equation?

    edit: W is coil weight and c_w is coil width
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  3. #3
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    To be honest i havent got a clue, i havent done math for ages since leaving school, it looks good to me.
    Basically im trying to work this out so i can enter it in to excel so in work i can enter the OD, ID & Width and it works out the weight for me.

    Thanks for your help so far
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  4. #4
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    Start by isolating that square root and tidy up some terms

    \dfrac{OD}{2000} = \sqrt{\dfrac{1000W}{24.66c_w}} + \dfrac{(ID)^2}{4 \cdot 10^6}

    \dfrac{OD}{2000}  - \dfrac{(ID)^2}{4 \cdot 10^6}= \sqrt{\dfrac{1000W}{24.66c_w}

    To make the next step easier I will multiply \dfrac{OD}{2000} by 2000/2000:

    \dfrac{2000(OD)}{4 \cdot 10^6}  - \dfrac{(ID)^2}{4 \cdot 10^6}= \sqrt{\dfrac{1000W}{24.66c_w}

    This gives the same denominator so I can sum them now:

    \dfrac{2000(OD) - (ID)^2}{4 \cdot 10^6}= \sqrt{\dfrac{1000W}{24.66c_w}

    Do you know how to continue?
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  5. #5
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    No i dont know how to continue, sorry im a complete novice
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  6. #6
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    Square both sides

    \left(\dfrac{2000(OD) - (ID)^2}{4 \cdot 10^6}\right)^2 \cdot 24.66c_w = 1000W

    Now it's just left to isolate W. If this is for an excel formula I wouldn't bother expanding out
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  7. #7
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    Thank you very much for your help so far i realy am greatful but could you expand it out like i first wrote it as i find it easier to understand?

    I know im being a lil cheeky but thanks for your help.
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  8. #8
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    Thread closed because the OP needs someone in person to do this for them, not anonymous internet helpers.
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