# Kinetic Energy

A moving object has the ability to do work. This ability is called the kinetic energy of the object.

The kinetic energy equals:

E = (1/2) . m . V2

where :
m : Mass (kg)
V : Speed (m/s)
E : Energy (Nm or J)

## Explanation

A moving object’s ability to do work can be seen in many examples: moving water powers a watermill, a thrown stone breaks a window etc.

The work of a moving object follows the definition W = F . d. We look at an object with speed V and mass m which is brought to zero speed by a force F. Because this force has an opposite direction to the speed, the force has a negative magnitude. Then, taking the integral from V to 0 (speed) over (F . dr) while substituting F with m.a, one will find that W = (1/2) . m . v2.

## Example

A stone of 0.5 kg is thrown at a speed of 5 m/s. How much kinetic energy does this moving object contain? E = (1/2) . m . v2 = (1/2) . 0.5 .  52
= 0.25 . 25 = 6.25 J.

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