We define the gravitational field to be the cause of the gravitational force.
We define it conveniently to be the force per unit mass
(13) |
The gravitational field at the surface of the earth is:
(14) |
This equation can be used to find , , , or , from any of the other three, depending on which ones you think you know best. is easy. is actually also easy to measure independently and some classic methods were used to do so long before Columbus. is hard! What about ?
Henry Cavendish made the first direct measurement of using a torsional pendulum and some really massive balls. From this he was able to ``weigh the earth'' (find ). By measuring ( measured between the centers) it was possible to directly measure . He got 6.754 (vs 6.673 currently accepted) N-m/kg. Not bad!