[imrehg]

Yup, that's exactly it:

- get the gravitational constant with these two known masses

- then can deduct the mass of the unknown Earth by its interaction with other masses (say the "g" gravitational acceleration value)

- then from the mass and the otherwise measured size of Earth the density pops out

More details in good ol' Wikipedia: https://en.wikipedia.org/wiki/Cavendish_experiment#Derivatio...

[UncleSlacky]

It's also a notoriously difficult experiment to perform. When I did it at university, the value of G I got was out by an order of magnitude - and that was considered a good result!