Consider the situation shown in the diagram below. 






The force, F will provide a moment (or torque) given by 








Assuming that there are no frictional (or other) forces opposing the motion
of the wheel, the torque will cause the angular velocity of the wheel to
change. 





Experiments show that the rate of change of angular velocity is
directly proportional to the torque. 





Expressing this result as an equation we have 





where the Greek letter
τ is used to represent moment or torque 

The constant is called the moment of inertia of the wheel and axle
and is given the symbol I so the equation
is written 





Recall that the inertia of a body is a measure of its reluctance to
change the state of its linear* motion. 

Similarly, the moment of inertia of a body is a measure of its
reluctance to change the state of its angular (rotational) motion. 



*Elsewhere, we might have just said "change the state of its motion". Now we
should be more specific. 
