Momentum
and Angular momentum

Ordinary** momentum** is a measure
of an object's tendency to move at constant speed along a
straight path. Momentum depends on speed and mass. A train moving
at 20 mph has more momentum than a bicyclist moving at the same
speed. A car colliding at 5 mph does not cause as much damage as
that same car colliding at 60 mph. For things moving in straight
lines:

When things move in curved paths, the idea of momentum can be
generalized as **angular momentum**. Angular momentum measures
an object's tendency to continue to spin or, in other words, the
angular momentum is the physical quantity which describes the
dynamics of objects that are spinning or revolving round an axis.
An ``object'' can be either a single body or two or more bodies
acting together as a single group. The angular momentum is
normally defined as:

#### angular momentum = mass × velocity ×
distance (from the point object is spinning or orbiting around)

Let's suppose the object (or group of objects) has no outside
forces acting on it (in a way to produce **torques** that
would disturb the angular motion of the object). In these cases,
we have **conservation of angular momentum.**

#### This means that the total amount of angular momentum does not
change with time no matter how the objects interact with one
another.

A simple example:

the angular momentum* L* of a dumbbell (two masses *m*,
connected by a massless bar of length *d*) freely spinning
on a plane with angular velocity *w* around the axis
perpendicular to this plane and passing through the center of the
bar, is computed as:

Since the dumbbell is a free spinning body, *L* is
conserved. If, for a particular reason, the distance *d *changes,
the angular velocity must change according to equation:

Hence, if d becomes smaller, the dumbbell must spin faster.
This is, for example, the reason why an ice-skater spins faster
when she keeps her arms closer to her body!!