# Rearranging the formula

• May 7th 2011, 08:18 PM
IBstudent
(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 ...

• May 7th 2011, 08:23 PM
topsquark
Quote:

Originally Posted by IBstudent
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 ...

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

Can you take it from here?

-Dan

Edit: $\displaystyle \frac{mv - mu}{v - u} \neq m - m$. 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:

$\displaystyle \frac{6 \cdot 3 - 6 \cdot 2}{3 - 2} = 6 - 6 = 0$
$\displaystyle \frac{6 \cdot 3 - 6 \cdot 2}{3 - 2} = \frac{18 - 12}{1} = 6$