# What is kinetic energy?

Kinetic energy is the energy of an object associated with its motion. It depends on how massive the object is and how fast it is moving.

In the case of translational motion, for example, a car driving on the road, the kinetic energy is calculated as

`K = 1/2 mV^2` , where m is the car's mass and V is the car's speed.

Another type of motion is rotational, such as a CD or a record rotating around its center. The kinetic energy associated with this motion is calculated as

`K = 1/2 I omega^2` , where I is the disk's moment of inertia (it indicates the distribution of the object around its center of mass) and `omega` is its angular speed, indicating how fast the disk is rotating.

When the object performs both types of motion, such as a ball rolling downhill, its kinetic energy would be the sum of its translational kinetic energy and its rotational kinetic energy.

An object can acquire kinetic energy, or lose kinetic energy,  when there is work done on it by an external force. The force can be conservative, such as gravity, or non-conservative, such as friction. The work-energy principle states that the change of the kinetic energy equals the net work done on the object:

`Delta K = W` , where W is the net work.

This means that if the net work is zero, the kinetic energy will not change and the object will be moving with constant speed. If the net work is positive, the kinetic energy will increase and the object will move faster. If the net work is negative, the kinetic energy will decrease and the object will slow down.

Please see the attached reference links for the further discussion of kinetic energy.