Demos: 1F-02 Stack of Washers

This demonstration illustrates the properties of inertia. A stack of brass washers is “unstacked” one at a time by striking the bottom washer quickly with a metal blade, knocking it out before the stack has a chance to move appreciably. It works less well as the stack gets shorter, because of the diminishing mass of the stack.

Directions: Pile the washers in a neat stack. A metal bar, the thickness of a washer, is bent in such a way that a portion of the blade can be made to lie flat against the table while holding the other end. Keeping the end of the blade flat on the table, quickly bring it against the bottom of the washer stack. The bottom washer flicks out and the remainder of the stack drops straight down.

Suggestions for Presentation: Illustrate the crucial aspect of this demo, namely that the force has to be large so that the washer to washer friction force will be very short-lived, so that the heavier stack will not have a chance to get moving before the force is gone. This can be done by moving the blade slowly and seeing the whole stack move.

Applications: Various mechanical operations where a piece can be removed from another larger one by a quick blows.

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