The same law keeps bicycles upright and makes neutron stars spin furiously
Spin something and it resists changes to that spin. That is angular momentum, and in an isolated system its total never changes. The rule explains why a moving bicycle stays upright, why rifled bullets fly true, why hurricanes curl into spirals and why collapsed neutron stars rotate at astonishing rates.
Angular momentum is the rotating counterpart of ordinary momentum, sometimes called moment of momentum. For a single particle it combines the particle's momentum with its position measured from a chosen point, which means that, unlike linear momentum, its value depends on where that reference point sits. The classic picture is the moment arm: the perpendicular distance from the origin to the particle's path, multiplied by its momentum. It carries both a size and a direction, and both are conserved.
For a spinning object, angular momentum is the product of its moment of inertia and its rotation rate. Moment of inertia is subtler than mass, because it depends not just on how much matter there is but on how that matter is arranged around the axis. The quantity is also additive: the angular momentum of a composite system is the sum of its parts.
Change requires a twist. Just as a force alters linear momentum, a torque alters angular momentum, and torque can be defined as its rate of change. Internal torques within a system always cancel, a rotational version of Newton's third law, so without an outside torque the total stays fixed. The change produced by a particular interaction is called angular impulse, occasionally nicknamed twirl. Conservation restricts how a system can move, though it does not dictate every detail.
Physicists separate two kinds. Spin angular momentum belongs to rotation about an object's own centre of mass; orbital angular momentum belongs to motion around some other centre. Earth has both: its orbit around the Sun gives about 2.66 × 1040 joule-seconds, while its daily rotation contributes only about 7.05 × 1033. Strictly, what is conserved is the total for the whole Solar System, since planets and Sun trade small amounts among themselves. Gyroscopes and flying discs exploit the same principle every day.
Source: Angular momentum
