Originally Posted by

**Ackbeet** Technically, the first equation is

$\displaystyle F=-\dfrac{G m_{1}m_{2}}{r^{2}},$

where $\displaystyle G$ is the gravitational constant (not 9.8 meters per second squared). The $\displaystyle W=mg$ equation is a useful approximation when everything is happening at more or less the same distances from the Earth's center. Essentially, you have

$\displaystyle g=\dfrac{G m_{\text{earth}}}{r_{\text{earth}}^{2}}.$

The first equation in this post is the more correct equation, especially when your distance away from Earth's center is changing by a lot.

Does that make sense?

Your computation of the mass is correct. What more can you do?