# Homework Help: Basic Equation Rearrangement

1. Nov 24, 2009

### Sharr-zeor

i fail miserably at rearranging equations. how would i rearrange GM/r^2 = v^2/r to make M the subject?

this is to do with centripetal force, and the mass of a planet a space probe is orbiting

after working out the answer to v^2/r (which is 1.35ms^-2) im left with GM/r^2 = 1.35ms^-2 i wasnt sure if it would be M = 1.35G/r^2....

as i said, i am terrible at rearrangements. any help would be appreciated

2. Nov 24, 2009

### Staff: Mentor

Well, per the PF rules (see the link at the top of the page), we can't just give you the answer.

So, what steps do you normally use to re-arrange equations? If you multiply both sides by the same thing, does the equality still hold? Divide both sides by the same thing? Multiply one side by 1? (using something like 1=x/x)

3. Nov 24, 2009

### Sharr-zeor

well i only need the eqation to work out the answer to my coursework question. i've taken the steps to find the necessary equation, i just cant rearrange it.

i dont know if im multiplying or dividing anything

oooh, if i multiply 1.35 by r^2 i get GM?
therefore 1.35r^2/G = M ?

if this is right im going to kick myself for not thinking aboutthe multiplication/division. i always forget the simple bits...

4. Nov 24, 2009

### Staff: Mentor

It's hard for me to check your work -- where did the 1.35 come from? It would be easier for us to check your work if you just left the equation symbollic until the end.

and leave it symbollic, what do you get for M = ?

5. Nov 24, 2009

### Sharr-zeor

leaving it symbolic i get (v^2 / r) r^2 / G = M ?

would that work out as v^2 r / G = M

v [squared] r [divided by] G [equals] M

6. Nov 24, 2009

### Staff: Mentor

Good job!

7. Nov 24, 2009

### Sharr-zeor

so v^2 r / G = M?

it seems so simple now >.<

thanks for the help, i guess i need to just go through it step by step instead of rushing to an answer :)