A ball swung on a string held by a sleeve.

Fictitious Forces: Centrifugal and centripetal acceleration. A qualitative demonstration of the relationship of m, v , and r. The sleeve is held in the hand, and the ball is swung in a circular motion. Pulling on the ring shortens the radius of the balls path and increases the velocity.
UCB Index: 
A+0+35
PIRA Index: 
1D50.10
Demo Diagram: 

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Comments

As always when you present a demo make sure you ask the participants questions before you demonstrate and after. In this case asking if the participant thinks the ball will speed up or slow down as the radius decreases is key. Depending on the age of the participant bring in examples of circular (or close to circular) motion like orbits or loop de loops in roller coasters. Be very careful that you control this demo at all times and don't let it strike anyone. Think about the concepts involved beforehand; having a friend ask you questions about it can be very revealing as to if you understand it well or not. Whenever you ask a question count to five (or at least three) in your head waiting for an answer before you stay anything else.