**Univeral law of gravitation:**

The mathematical formula that relates the pull of gravity between two massive objects that are some distance apart. F_{g}=Gm_{1}m_{2}/d^{2}

**Gravitation constant: **

A proportionality constant that relates the strength of gravitational attraction in Newton’s law of universal gravitation. G=6.67x10^(-11) Nm^{2}/kg^{2} This value always remains constant.

**Gravitational field: **

The map of influence that a massive body extends into space around itself.

**Rotational speed: **

Number of rotations or revolutions per unit of time, often measured in rpm, revolutions per minute. Linear speed is the straight path distance moved per unit of time, also referred to as tangential speed.

**Centripetal acceleration:**

The acceleration that describes the change in direction for an object in a circular path.

**Centripetal force: **

A center seeking force for an object moving in a circular path.

**Centrifugal force:**

An apparent, but nonexistent, outward pointing force for an object moving in a circular path. A rotating object may seem to be pushed outward, but actually must be pulled inward in order to maintain any circular path.