# gravity

From OrbiterWiki

**Gravity** is the tendency of objects with mass to accelerate toward each other.

**Newton's law of universal gravitation** states the following:

Every point mass attracts every other point mass by a force directed along the line connecting the two. This force is proportional to the product of the masses and inversely proportional to the square of the distance between them:

F= -GMm/r^{2}

where:

Fis the magnitude of the gravitational force between the two point massesGis the gravitational constantMis the mass of the first point massmis the mass of the second point massris the distance between the two point masses

Assuming SI units, *F* is measured in Newtons (N), *M* and *m* in kilograms (kg), **r** in metres (m), and Newton's gravitational constant *G* is approximately equal to 6.67 × 10−11 N m2 kg−2 (Newtons times metres-squared per kilogram-squared).

It can be seen that the force *F* is always negative. This sign convention is consistent with its electromagnetic equivalent *Coulomb's Law*, where a positive force means repulsion between two charges.

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