Re: Relative velocity problem....

[Maurice Barnhill <mvb@UDel.Edu>]

von Philp wrote:
> Please help me get a concept across to my students.  Here is the
> problem:
>
> A person rides in the back of a pick-up truck moving forward at +20
> mph
> (relative to ground).  The person throws a brick toward the back of
> the
> truck at -5 mph (relative to the truck).  What is the resultant
> velocity of
> the brick relatvive to the ground?
>
> ...
> And short of going to the parking
> lot
> with a truck and a brick (maybe basketball -- safer) I am at a loss
> getting
> them to believe this.

   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.  In
addition, you can come back later and talk about the forces on the
marble when you start the eraser moving.

> Ralph von Philp
> Boonsboro High School, MD


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