Rotating Frames of Reference

      
When a ball is placed on a frictionless rotating surface, the ball moves outward.  Although the ball has no additional force applied, it does have an initial velocity. 

This is a noninertial frame of reference because Newton's Laws of Motion do not apply.  

When observed by people on the ground, the ball continues on the of the initial velocity at the time of release. 

This in an inertial frame of reference because Newton's Laws of Motion apply.

This is the same path a rock released from an old fashion sling takes.  If the rock is to go forward, the warrior must release the rock when it is at his side.  If he releases the rock when it is in front of him, it will go to his side.

Although Newton's Laws of Motion do not apply, calculations can be simplified by by making use of a trick.  

Use equation F = ma as if a force equal to mv²/r  or m²r is acting radially outward on the object in addition to any other forces acting on the object.  This pseudoforce or fictitious force is called centrifugal force.  Remember, it is not a true force.  It is not necessary in calculations if we are using an inertial frame of reference.  It is also called an inertial force because it is only necessary in a noninertial reference frame.