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Re: Relative velocity problem....



Maurice Barnhill wrote:

A quicker way to do the truck demo is to put a marble on an eraser
and pull the eraser forward along a desk or table. The students can
easily see that the marble is moving backward with respect to the
eraser, or, by concentrating on the table, that it is going forward
with respect to the table, especially after it rolls off the eraser.

At FIRST, this sounds like a nice, convincing demo .... but.....

If you try it, you will find that a blackboard eraser is so short that it
is difficult for students to "easily see" the direction that the marble
is moving with respect to the table. Everything happens in a second or
two. Also, as the ball rolls backward
and leaves the eraser, it continues to roll backwards as the eraser
continues to move in the opposite direction with respect to the eraser
and ALSO WITH RESPECT TO THE TABLE.

Herb Gottlieb from New York City
(Where physics students are a little harder to convince).