Re: Relative velocity problem....

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

On Wed, 10 Sep 1997, 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?
>
> ...
>
> Any ideas?

How about working with speeds of 20 ft/s and 5 ft/s and asking the
students to draw three successive pictures (one directly below the other) 
of the truck, the brick, and a person standing on the ground at the
instant the brick is thrown as well as one and two seconds later.  Have
them check 1) that the three pictures of the person on the ground line up
vertically (so that the pictures are drawn from the vantage point of that
person) and 2) that the brick is five feet behind the truck in the second
picture and ten feet behind the truck in the third (so that it is, indeed,
moving away from the back of the truck at 5 ft/s.) 

John
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