Homework Help: Equation creating

1. Nov 28, 2016

Ugnius

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. Nov 28, 2016

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?

3. Nov 29, 2016

Ugnius

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4. Nov 29, 2016

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?

5. Nov 29, 2016

Ugnius

Yes i do, i get lost when there are more than 3

6. Nov 29, 2016

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.

7. Nov 29, 2016

CWatters

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"

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.

8. Nov 29, 2016

Ugnius

but if i use efficenty precent (n) should i use the number which is 0.28 or precent 0.28*100% = 28%?

9. Nov 29, 2016

Staff: Mentor

Use the decimal equivalent -- 0.28 -- not the percent value.

10. Nov 29, 2016

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.