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.

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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.
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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.
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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.
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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.
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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.
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Angular momentum must be conserved in this
situation, so the star's rotation speed must
increase. Some neutron stars rotate in seconds.
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A rotating object also has kinetic energy due to
its rotation.
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This is the nature of the need for the infamous
frictionless, massless pulley used in physics
problems.
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A machine is a device used to multiply forces or
to change the direction of forces.
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The six simple machines are the lever, the
pulley, the wheel and axle, the inclined plane,
the wedge, and the screw.
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The ratio of input force to output force for a
machine is the mechanical advantage.
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Efficiency of a machine is the ratio of useful
work output to total work input.
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Efficiency is also the ratio of actual
mechanical advantage to theoretical mechanical
advantage.
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These two values in the ratio would be the same
and efficiency would be 100 if it were not for
friction.
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