Re: Relative velocity problem....

[Bob Sciamanda <Sciamanda@worldnet.att.net>]

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?
>
> 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

Hi Ralph,
Get an electrically powered toy car going on a throw rug.  Then pull the
rug at a greater velocity in the direction opposite to the car's
original velocity.  Time the car's motion between marks a) on the rug
and b) on the floor or wall; also time the rug's motion between marks on
the floor.  Thus measure three velocities: car relative to rug, car
relative to floor (or wall), and rug relative to floor.  You can take a
videotapes for analysis.

If you can find a "moving sidewalk" in a mall or airport, or a
treadmill, you can use people in place of the car.
-- 
Bob Sciamanda		sciamanda@edinboro.edu
Dept of Physics		sciamanda@worldnet.att.net
Edinboro Univ of PA	http://www.edinboro.edu/~sciamanda/home.html
Edinboro, PA		(814)838-7185