### 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