If you have an old chalk board or white board that is magnetic, creating a marble run from recycled materials is really simple.
Cut plastic, or even cardboard, tubing in half lengthwise to create the basic "ramp" for the marbles.
Hot glue magnets on the back so each ramp will stick to the chalk board.
To create a "pinwheel" within a tube, use a small nail to poke two holes opposite each other in a short length of tube.
Cut a piece of straw slightly smaller than the diameter of the tube and use short pieces of masking tape to create "spokes."
Two strips of tape make two spokes.
Secure the straw inside the tube by stringing it on to a short piece of skewer affixed into the two nail holes.
As the marble falls vertically, the pinwheel will spin.
Attaching several magnets to the a piece of flexible, foam tube will allow you to create different curves.
A magnet attached to a plastic cup can create an end point to catch the marble.
PVC pipes make great tubes as well.
A fun thing to experiment with is creating a V shape with two straws. Even when parallel, the marble will move along with the widening V.
The upside down tops of soda bottles will make the marble swirl around before it drops.
Adding thumbtacks, or even strips of hot glue or sandpaper will add some friction to the course.
Since everything is magnetic, this marble run can be rearranged over and over again.
The marble is moved down the ramp by gravity. An attraction, or gravitational force exists between the marble and the earth. Since the earth is much more massive than the marble, the marble wants to get as close as possible to the earth. If you drop a marble out of your hand, it falls straight down to the earth. In the magnetic marble run, the marble has to go through the different obstacles to get as close as it can to the earth.
The marble will speed up or slow down depending on the obstacle. Pipes and tubes that are nearly flat and ones with higher friction will slow down the marble, while steep obstacles will low friction will speed up the marble. When an object is sitting on a flat surface, the only force acting on it is gravity, which is always downward. The object cannot move any farther down on a flat surface since gravity is not strong enough to push it through the surface. When an object is sitting on an incline, or ramp, the downward force of gravity