#### Transcript A moving object has a tendency to keep moving, this is momentum

```A moving object has
a tendency to keep
moving, this is
momentum. A rotating
object has a tendency
to keep rotating; this is
angular momentum.
If the net external torque
on an object is zero, the
angular momentum
does not change.
This is the law of
conservation of
angular momentum.
When an ice skater
pulls his arms in
closer to his body, he
decreases the length
of the torque arm,
which decreases the
moment of inertia.
Angular momentum is
equal to moment of inertia
X angular speed. If the
moment of inertia is
decreased (by pulling the
arms in) the angular
speed must increase.
If you walk to the middle of a
spinning merry-go-round, it
will speed up.
Neutron stars are formed by
the collapse of a star's matter
following a supernova.
Angular momentum
must be conserved in
this situation, so the
star's rotation speed
must increase. Some
neutron stars rotate in
seconds.
A rotating object
also has kinetic
energy due to its
rotation.
This is the nature of
the need for the
infamous frictionless,
massless pulley used
in physics problems.
A machine is a
device used to
multiply forces
or to change the
direction of forces.
The six simple
machines are the lever,
the pulley, the wheel
and axle, the inclined
plane, the wedge, and
the screw.
The ratio of input
force to output
force for a machine
is the mechanical
Efficiency of a
machine is the
ratio of useful
work output to
total work input.
Efficiency is also
the ratio of actual