### Gravitational Force

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

where is the distance between the centroids of the masses and at time , and is the

*gravitation constant*.

^{B.2}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.

**Next Section:**

Hooke's Law

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Mass