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## Homework Statement

A moon orbits a planet of unknown mass with a period of 1.8 days. If the radius of the moon's orbit is 420,000,000 meters, find the mass of the planet.

## Homework Equations

For the speed of the moon I got: v=2 x pie x r / v

2 x pie x 420000000 / v = 1.8 days or 155520 seconds since speed is m/s.

v= 16968.5 m/s.

For the centripetal acceleration of the moon I got this: a= v^2/ r

16968.5 ^2 / 420000000 = .69 s

Now for the mass of the planet I am not quite sure how to figure out. If I know the speed and centripetal acceleration for the moon how do I use this to get the mass of the planet orbiting the moon??

## The Attempt at a Solution

My attempt but was un successful:

v^2= G (Me/r)

16968.5 m/s^2= 6.67E-11 ( Me/420000000)

1.03E10