# Why velocity increases when radius decreases

1. Sep 15, 2007

### vivinisaac

pls explain what is angular momentum
and if possible explain why velocity increases when radius decreases (not mathematically)

2. Sep 15, 2007

### rcgldr

Linear momentum is equal to speed x mass.

Angular momentum is basically the same, except the mass is traveling in a circular path, it's relative to the angular velocity (speed of rotation) x mass x radius. The speed of the mass is equal to the angular velocity x radius, and if mass and angular momentum are constant, than a decrease in radius requires an increase in angular velocity, while the speed of the mass remains constant (same speed in a smaller circler means a faster rate of rotation).

3. Sep 16, 2007

### Loren Booda

Also, angular momentum is a conserved quantity, having the units of gm-cm2/sec (or erg-sec) in the cgs system. Angular momentum is the cross product of a momentum vector and its radial vector, thus oriented perpendicular to both.