Kinetic energy
Kinetic energy (also called
vis viva, or
living force) is
energy possessed by a body by virtue of its
motion. The kinetic energy of a body is equal to the amount of
work needed to establish its velocity and rotation, starting from rest.
In words the above equation states that the kinetic energy (E_{k}) is equal to the integral of the
dot product of the
velocity (
v) of a body and the
infinitesimal of the body's
momentum (
p).
For nonrelativistic mechanics, the total kinetic energy of a body can be considered as the sum of the body's translational kinetic energy and its rotational energy, or angular kinetic energy:
where:
 E_{k} is the total kinetic energy
 E_{t} is the translational kinetic energy
 E_{r} is the rotational kinetic energy
For the translational kinetic energy of a body with mass m, whose
centre of mass is moving in a straight line with
linear velocity v, we can use the
Newtonian approximation:
If a body is rotating, its
rotational kinetic energy or
angular kinetic energy is calculated from:
 ,
where:
Relativistic mechanics
In Einstein's relativistic mechanics, (used especially for nearlight velocities) the kinetic energy of a body is:

 E_{k} is the kinetic energy of the body
 v is the velocity of the body
 m is its rest mass
 c is the speed of light in a vacuum.
 γmc^{2} is the total energy of the body
 mc^{2} is the rest mass energy.
It is an edifying exercise to show that the ratio of this relativistic kinetic energy to the Newtonian kinetic energry given by (1/2)
mv^{2 approaches 1 as v approaches 0, i.e.,
This can be done by the techniques of firstyear calculus.
Relativity theory states that the kinetic energy of an object grows towards infinity as its velocity approaches the speed of light, and thus that it is impossible to accelerate an object to this boundary.
Where gravity is weak, and objects move at much slower velocities than light (e.g. in everyday phenomena on Earth), Newton's formula is an excellent approximation of relativistic kinetic energy.
Heat as kinetic energy
Heat is a form of energy due to the total kinetic energy of molecules and atoms of matter. The relationship between heat, temperature and kinetic energy of atoms and molecules is the subject of statistical mechanics. Heat is more akin to work in that it represents a change in internal energy. The energy that heat represents specifically refers to the energy associated with the random translational motion of atoms and molecules in some identifiable matter within a system. The conservation of heat and work form the first law of thermodynamics.
See also
Recoil
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