Kinetic Energy: Definition, Formula, Types (W/ Examples)

Kinetic energy is the energy of motion; any moving object has kinetic energy. It is one of two big buckets that describe mechanical energy; the other is potential energy, which is a form of energy that is stored.

Something can have both potential and kinetic energy, and these forms of energy can transform back and forth so long as the total energy never changes. This is because of the ​law of conservation of energy​, which states that the total energy in a closed system remains constant.

Consider a roller coaster going down a hill. At the bottom, its velocity is the greatest – as is its kinetic energy. Halfway back up to its highest point, it has nearly equal amounts of gravitational potential energy and kinetic energy, and then at the top, when it may be barely moving at all, most of its energy is potential energy. And yet at all points on its path, the total energy remains the same.

Mechanical kinetic energy of an object of mass ​m​ moving with velocity ​v​ is given by the formula:

\(KE_{mech}=\frac{1}{2}mv^2\)

The SI unit for ​KE​ is the Joule (J) where 1 J = 1 Nm. The heavier the mass and the faster it is moving, the more kinetic energy it has, but it depends linearly on the mass while scaling with the square of the speed.

​Mechanical kinetic energy​ is associated with the mechanical motion of an object. It can have translational (linear) kinetic energy and/or rotational (spinning) kinetic energy. For example a ball rolling across the floor has both translational and rotational kinetic energy.

​Radiant kinetic energy​ is energy in the form of electromagnetic radiation. You may be most familiar with visible light, but this energy comes in types we can't see as well, such as radio waves, microwaves, infrared, ultraviolet, X-rays and gamma rays. It is energy carried by photons – particles of light.

Photons are said to exhibit particle/wave duality, meaning they act both like a wave and a particle. They differ from regular waves in a very critical way: They don't require a medium through which to travel. Because of this, they can travel through the vacuum of space.

​Thermal kinetic energy​, also known as heat energy, is the result of the molecules in a substance vibrating. The faster the molecules vibrate, the greater the thermal energy and the hotter the object. The slower the vibrations, the colder the object. At the limit where all motion stops, the temperature of the object is absolute 0 on the Kelvin scale. Temperature is a measure of the average translational kinetic energy per molecule.

Other forms of energy are often transformed into thermal energy as a result of frictional or dissipative forces. Think of rubbing your hands together to warm them – you are converting mechanical kinetic energy into thermal energy!

With ​sound​ and ​wave kinetic energy​, a disturbance travels through a medium. Any point in that medium will oscillate in place as the wave passes through – either aligned with the direction of motion (a ​longitudinal wave​) or perpendicular to it (a ​transverse wave​), such as is seen with a wave on a string.

While the points in the medium oscillate in place, the disturbance itself travels from one place to another. This is a form of kinetic energy because it is the result of a physical material moving.

A sound wave is a longitudinal wave. That is, it results from compressions and rarefactions in air (most commonly) or another material. A ​compression​ is a region in which the medium is compressed and more dense, and a ​rarefaction​ is a region that is less dense.

​Electrical kinetic energy​ is the kinetic energy associated with a moving charge. It is the same mechanical kinetic energy 1/2mv²; however, a moving charge also generates a magnetic field. That magnetic field, just like a gravitational or electric field, has the ability to impart potential energy on anything that can "feel" it – such as a magnet or another moving charge.

When moving charge makes its way through a circuit, the elements in the circuit allow for the associated energy to be converted to light energy, or other forms as the circuit is used to power various electronic devices.