Reactionary Roadbed

Radio controlled car moves one way while road moves the other.

What it shows:

We tell our students that, when a car drives down the road, the road and the Earth move in the opposite direction, albeit imperceptibly. This demonstration is a realization of that concept, made possible (and perceptible) by the fact that the road is not attached to the Earth.

radio controlled car

How it works:

The automobile is a radio-controlled, battery powered, model car. 1 The "road" is a 16" × 8' long piece of 1" thick foam insulation 2 and is supported by a dozen empty Coke™ cans. The light cans have a very small moment of inertia, roll easily, and give very little resistance to the motion of the road they support. The car and road each weigh 2 lbs (almost 1kg). When the car tries to move forward, the road moves backward and vice versa. Since the road is not very long, one has to repeatedly alternate between forward and reverse, the result being no net motion.

Setting it up:

Although there's room on top of the lecture benches, it's best to set this up on the floor because the car doesn't always stay on the road and we're not demonstrating free-fall over a cliff. Spread out two rows of six Coke cans to support the road.

radio controlled carradio controlled car


Lots of fun.

1 Radio Shack digital control Turbo
2 Celotex™ insulating sheathing. It's backed by aluminum foil on both sides which makes stiff - that's important in this application.