# Homework Help: Just need some clarification

1. Aug 12, 2006

### highc

I'm working through a problem that was posted here a while ago (https://www.physicsforums.com/showthread.php?t=115170). maltesers posted the equation (Gm)/(r-S)^2 = (GM)/r^2, so far this makes perfect sense, he then goes on to solve for S with S = r - (mr^2)/M, and it is here that he loses me. How did we eliminate the ^2 to go from (r-S)^2 to S = r - ...?

I've been out of the math game a little too long!

2. Aug 12, 2006

### neutrino

That equation is actually incorrect. It should have been

$$\frac{Gm}{(r-S)^2} = \frac{GM}{S^2}$$

3. Aug 12, 2006

### George Jones

Staff Emeritus
This not the only thing wrong.

Look at units. In maltesers' last equation, S is a length, while the last term on the right has units of length squared.

4. Aug 12, 2006

### neutrino

Now that it's known that the original equation is wrong, why bother about what was derived from it. :tongue2:

Thanks for pointing that out.

5. Aug 12, 2006

### highc

Thanks alot, I wasted way too much time on this one.