[[Work Energy and Power]]

Formula is: $$K=\frac{1}{2}mv^2$$
$K$ in joules
$m$ in kg
$v$ in m/s

* Kinetic energy is scalar, only magnitude and never negative.
* Because velocity is squared, changing speed affects energy more than changing mass.
* For a system of objects, total kinetic energy is the sum of each object's kinetic energy.
* Different observers in different reference frames can calculate different kinetic energies.

If a dude standing on earth sees a 1kg ball travelling at 10m/s, he'll calculate it as 50J. If a dude taped to the ball calculates it he'll calculate 0J. If a theoretical god looked at it from a universal standpoint he'd probably calculate like $1*10^6$ Joules due to Earth orbiting the sun and the solar system orbiting the Milky Way.

In a collision, total energy before and after is consistent in the observers frame.