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Relative velocity problem....




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?

I know that the brick will move forward at + 15 mph relative to the ground.
My students can't really believe this. Oh, they will accept it simply
because I told them that is what happens, but they do not really believe
that you can throw something backwards yet have it really be moving
foreward relative to the ground. I have no modeling software (i.e.
Interactive Physics) at the moment. 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.

They accept that if they throw the brick backwards at -20 mph the brick
will have no velocity relative to the ground and will simply fall. And
they accept that if they throw the brick straight up it will land back in
the truck (after some simple demos). Yet they will not accept the brick
moving forward. I know, I know.... if they accept the others, it is a
logical conclusion. But they don't see it.

Any ideas?

Ralph von Philp
Boonsboro High School, MD