Homework Help: Need serious help to understand how to rearrange scientific formula

1. Jan 23, 2012

pok243

I am currently doing AS level physics at school. However I still can't rearrange formula for questions. I literally have no idea on how to go about doing it.

For example, a simple formula like R=V/I

I tried to rearrange it so I could find I. I tried using an example I found on this forum, so I did it like this:
RxV=V/IxV=I so therefore I=RxV. However I know this is completely wrong so can someone please help me to understand how to rearrange formulas. I know that you are meant to do the opposite of what is in the forula e.g if there is division, you times it and vice versa. So please someone help me before I fail really badly.

2. Jan 23, 2012

Staff: Mentor

rearranging formulas is basic algebra so for example solving for X in the formula Z=XY

multiply both sides by 1/Y to get Z/Y = XY/Y = X Z/Y= X with the caveat that Y=/= 0

or for another example Z = X / Y how would you solve for Y?

3. Jan 23, 2012

HallsofIvy

Its hard to know where to start without knowing what you do know. I am going to assume that you are at least able to do arithmetic and know that, for example, 5*5= 25 while 5/5= 1. That is, a a number divided by itself is 1. What you have: RxV= V/IxV= I is wrong for two reasons. V times V is $V^2$, not 1 and the I remains in the denominator- $(V/I)xV= V^2/I$, not I. Instead you should start by noting that, in the original formula, the value you want to solve for, I, is in the denominator. There are two ways to handle that:
1) Multiply both sides by $I$
(RxV)(I)= (V/I)xI= V.
2) Invert the fractions (if course, we have to think of RxV A as the "fraction" RxV/1)
1/(RV)= I/V

If you did (1) then you can proceed by dividing both sides by RxV: I= V/(RV)= 1/R.
If you did (2) then you can proceed by multiplying both sides by V: V(1/RV)= 1/R= I.

4. Jan 23, 2012

You might want to start simple. Khan Academy has some basic videos on Simple equations that you might want to check out: Start here and watch at least up until the video entitled "Equations 3". They are each only 10 minutes (about).

5. Jan 24, 2012

Staff: Mentor

1) NEVER utter the word or operation that some people call "transpose". It is practically guaranteed to block you from ever mastering the process of rearranging an equation.

2) Don't use x for multiplication. You can usually omit the symbol since two pronumerals adjacent are by convention multiplied. If you feel lost without a multiplication sign in handwriting, use a heavy central dot. Sooner or later you will confuse x for a lowercase x.

3) Don't write this sign "/" for division, use a horizontal line and write the divisor underneath, it is much much clearer. (You'll grow to like it and appreciate its value here.)

4) Feel free to experiment wildly. Paper and pencils are cheap. Start off by writing V=IR and multiply or divide both sides by one or more terms, just to see what the outcome is. I always say maths is the ultimate science in which to experiment: all you need is a pencil, paper, calculator and your brain, and you have a complete laboratory there in which to experiment with complete safety. Now you can't do that in a chemistry laboratory!

5) Set yourself some problems, check your answer by evaluating with some numbers (use
your calculator). For example, take the formula that gives temperature in degrees C when you know it in degrees F, and rearrange it to give degrees F when you know degrees C.

C = 5(F - 32)/9

Good luck! Practice makes perfect.