# Newton's version of kepler's law

1. Feb 27, 2005

### Moe_the_Genius

Question:
Use Newton's version of Kepler's third law to find the mass of Earth. The moon is 3.9*10^(8) m away from Earth, and the moon has a period of 27.33 days.

I just have trouble with using the correct units...thanks for any help!

2. Feb 27, 2005

### James R

Convert days to seconds, and your data will be in standard SI units.

3. Mar 28, 2005

### Andrew Mason

$$M_m\omega^2R = \frac{GM_eM_m}{R^2}$$

$$M_e = \frac{4\pi^2R^3}{GT^2}$$

T is the period in seconds. R is in metres.

AM