We are all familiar with the force of gravity. It is a fundamental observed property of our universe that any two masses and experience an attracting force given by the formula
The law of gravitation Eq. (B.2) can be accepted as an experimental fact which defines the concept of a force. B.3 The giant conceptual leap taken by Newton was that the law of gravitation is universal--applying to celestial bodies as well as objects on earth. When a mass is ``dropped'' and allowed to ``fall'' in a gravitational field, it is observed to experience a uniform acceleration proportional to its mass. Newton's second law of motion (§B.1) quantifies this result.