# Distance between centre of earth and centre of moon

So here's the question:

Calculate the approximate distance between the centre of the Earth and the centre of the moon. You may use the mass of the Earth as $6\times10^{24} kg$ and $G = 6.67\times10^{-11} Nm^2kg^{-2}.$

I can't work this out, at least. This is what I've done.

$F=\frac{GMm}{r^2}=ma$ where $a=g$, so $r=\sqrt{\frac{Gm}{g}}$ but I feel like $a\neq g$ as it is far away?

• The sign on the exponent of 10^11 in G should be negative. Apr 17 '18 at 13:52
• The key to solving this problem is knowing how long a (sidereal) month is.
– JEB
Apr 17 '18 at 13:57
• So one month is the period of the moon? Apr 17 '18 at 14:04
• @DanD'silva Yes. 1 Sidereal month.
– JEB
Apr 17 '18 at 15:17