Formula: Relativistic Energy-Momentum Relation
$$W ~=~ \sqrt{W_{0}^2 ~+~ (p \, c)^2}$$
$$W ~=~ \sqrt{W_{0}^2 ~+~ (p \, c)^2}$$
$$W_0 ~=~ \sqrt{ W^2 ~-~ (p\,c)^2 }$$
$$p ~=~ \sqrt{ \frac{W^2 ~-~ {W_0}^2 }{ c^2 } }$$
Energy
$$ W $$ Unit $$ \mathrm{J} $$
Relativistic total energy of a body, which is also valid at high velocities.
Rest energy
$$ W_0 $$ Unit $$ \mathrm{J} $$
Rest energy is the energy of the body in its resting frame of reference. The rest energy of a photon is zero.
Momentum
$$ p $$ Unit $$ \frac{\mathrm{kg} \, \mathrm{m}}{\mathrm{s}} $$
Relativistic momentum of the body.
Speed of light
$$ c $$ Unit $$ \frac{\mathrm m}{\mathrm s} $$
Speed of light is a physical constant and indicates how fast light travels in empty space (vacuum). It has the following exact value in vacuum:
$$ c ~=~ 299 \, 792 \, 458 \, \frac{ \mathrm{m} }{ \mathrm{s} } $$