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 . V
2
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 . v
2.
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 . v
2 = (1/2) . 0.5 . 5
2
= 0.25 . 25 = 6.25 J.