Rev up the car and send it up or
across the chalkboard, while moving the board vertically at a constant
velocity. Note that the car motor has two speeds and the slower
one is
preferable here; you can switch between them by pressing down on the
car.
The car will not make it across the entire board, so to illustrate the
"crossing the river" problem, send it across the demonstration table on
a piece of poster board that is moving at the appropriate speed.