Kinetic Energy

E_{K}

Units:J

The first type of energy we would like to consider is kinetic energy, which is the energy an object has due to the fact that it is moving. The kinetic energy of an object is given by the formula

E_{K} = \frac{1}{2} m v^{2}

where m is the mass of the object, and v is the speed. Unlike momentum, kinetic energy (like all energy) is a scalar not a vector, and it is never negative.

The SI unit of energy is the Joule (J), where 1J is 1 Newton-meter. When a 100kg person walks at a normal pace (1 m/s) they have a kinetic energy of $E_{K} = \frac{1}{2} (100 kg) (1 m / s)^{2}$ or 50 joules.

Remember that the speed of a particle is related to its velocity by $v = | \vec{v} | = \sqrt{v_{x}^{2} + v_{y}^{2} + v_{z}^{2}}$ so an object's kinetic energy can be written in terms of velocity's components as $E_{K} = \frac{1}{2} m (v_{x}^{2} + v_{y}^{2} + v_{z}^{2})$

An object's kinetic energy can only change if a) work is done on the object, or b) that energy is changed into another form.