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Homework Help: Equation creating

  1. Nov 28, 2016 #1
    1. The problem statement, all variables and given/known data
    I come out with problem that i can't remake equation , what i mean by that is if i have equation with more then 3 units i don't know how to replace them

    2. Relevant equations

    Lets say we use piramid law or 6 3 2 on counting mass
    M(6) = q(3) * V(2) and bla bla bla
    but if we have n(eff.precent) = Nt/qm how to get N?
    3. The attempt at a solution
    Have no clue
  2. jcsd
  3. Nov 28, 2016 #2


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    Staff: Mentor

    Hi Ugnius,

    I'm having difficulty interpreting your question. Are you trying to isolate (solve for) a particular variable in a given expression? Something else? I don't understand the reference to "piramid law" (presumably "pyramid law"), is it a significant part of the problem? Can you provide more detail in your description?
  4. Nov 29, 2016 #3
    Im talking about this piramid method , works for every equation with 3 units

    Attached Files:

  5. Nov 29, 2016 #4


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    Staff: Mentor

    Okay, so the "piramid law" is a graphical method to represent the algebraic relationships between three variables, such as your M, D and V:

    given ##M = D \times V## then ##D = M/V## and ##V = M/D##.

    If you want to "solve" for any one variable, just place your thumb over it on the chart and read off the required operation between the other two variables by their positions in the chart. The chart is just a way to represent the underlying algebra. It might be a handy device for beginners learning abut Ohm's Law, for example:


    So are you asking if there are similar graphical devices for situations where there are more than three variables, or are you simply asking how, in general, to isolate a given variable when the equation involves more than three variables?
  6. Nov 29, 2016 #5
    Yes i do, i get lost when there are more than 3
  7. Nov 29, 2016 #6


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    Staff: Mentor

    The answer then is to review the basic rules of algebra, how to apply identical operations on both sides of an equation so that it remains in balance.

    This is not a matter of physics, it is basic math. The topic is better suited to the Precalculus Mathematics Homework forum, so I shall move the thread there.
  8. Nov 29, 2016 #7


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    Science Advisor
    Homework Helper
    Gold Member

    My children have just been taught the following rule/method for rearrangeing equations....

    "Whatever you do to one side of an equation you must also do to the other"

    For example if we start with your..

    n = Nt/qm

    then to get it in the form

    N = ???

    we must "get rid of" the t/qm on the right leaving just the N. Lets start by moving the "t" over to the left. To do that we divide both sides by t and you get

    n/t = Nt/tqm

    now t/t = 1 so the t on the right "cancels" to give

    n/t = 1*N/qm = N/qm

    Now we need to move the qm so this time we multiply both sides by qm to give..

    nqm/t = Nqm/qm

    Now the qm on the right cancels to give...

    nqm/t = N
    which is the same as
    N = nqm/t

    With practice you can do this in one step by multiplying both sides by qm/t.
  9. Nov 29, 2016 #8
    but if i use efficenty precent (n) should i use the number which is 0.28 or precent 0.28*100% = 28%?
  10. Nov 29, 2016 #9


    Staff: Mentor

    Use the decimal equivalent -- 0.28 -- not the percent value.
  11. Nov 29, 2016 #10


    Staff: Mentor

    In my opinion, this is silly, as it attempts to replace thinking by rote memorization.
    In the equation V = IR, if you want to isolate I (solve for I), divide both sides of the equation by R. This results in V/R = I.
    If you want to solve for R, divide both sides of the equation by I, resulting in V/I = R.
    I've never seen any visual devices with more than three variables. In an equation with four or more variables, the use of basic algebra techniques is your best bet.
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