The kinetic energy of an object is the
extra energy which it possesses due to its motion. It
is defined as the work needed to accelerate a body of
a given mass from rest to its current velocity. Having
gained this energy during its acceleration, the body maintains
this kinetic energy unless its speed changes. Negative
work of the same magnitude would be required to return
the body to a state of rest from that velocity.
Kinetic energy can be best understood
by examples that demonstrate how it is transformed from
other forms of energy and to the other forms. For example
a cyclist will use chemical energy that was provided by
food to accelerate a bicycle to a chosen speed. This speed
can be maintained without further work, except to overcome
air-resistance and friction. The energy has been converted
into the energy of motion, known as kinetic energy but
the process is not completely efficient and heat is also
produced within the cyclist.
The kinetic energy in the moving bicycle
and the cyclist can be converted to other forms. For example,
the cyclist could encounter a hill just high enough to
coast up, so that the bicycle comes to a complete halt
at the top. The kinetic energy has now largely been converted
to gravitational potential energy that can be released
by freewheeling down the other side of the hill. (Since
the bicycle lost some of its energy to friction, it will
never regain all of its speed without further pedaling.
Note that the energy is not lost because it has only been
converted to another form by friction.) Alternatively
the cyclist could connect a dynamo to one of the wheels
and also generate some electrical energy on the descent.
The bicycle would be traveling more slowly at the bottom
of the hill because some of the energy has been diverted
into making electrical power. Another possibility would
be for the cyclist to apply the brakes, in which case
the kinetic energy would be dissipated through friction
as heat energy.
