Re: Relative velocity problem....

[Maurice Barnhill <mvb@UDel.Edu>]

Since my post,  "Surendranath Reddy.B." <bsreddy@hd1.vsnl.net.in> has
described a demo that I like better than mine.  However, I checked mine
with a steel ball from the demo room and with chalk; the steel ball
worked, but on some trials the chalk stopped dead on the table, so I
guess that chalk or marble could easily go backward.  If you have
paper-backed (chalkboard) erasers, two erasers sliding with their backs
in contact work fine.

As far as whether an eraser is long enough, I guess it depends on the
size of the class.  I wouldn't try it in our 240-student auditorium, but
it seems to work well enough with a class of 20-something.

One advantage of a demo using objects that are readily available
(provided you choose them a bit better than I did in mentioning marbles
for this one) is that a skeptical student can try the demo at home.

herbgottlieb@juno.com wrote:
> 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).

-- 
Maurice Barnhill, mvb@udel.edu                
http://www.physics.udel.edu/~barnhill/          
Physics Dept., University of Delaware, Newark, DE 19716