20. Galilean Relativity

A toy LGB train has a flat car with a vertically mounted gun, actuated by a flashlight. When the train is at rest, the projectile ball is shot vertically upward and falls back down into the gun muzzle. If the train is moving along a straight section of track, the ball is still caught by the gun, even though the trajectory of the ball is now a parabola. The train can even be sent through a tunnel so the ball is fired up before the flat car enters the tunnel, passes over the tunnel, and is caught as the flat car exists the tunnel.

If the gun is fired as the train is rounding a curved section of track, the ball will not be caught, illustrating the restriction of special relativity to inertial frames.

This is a fairly elaborate demonstration, and several people are required to set it up, the set-up time often running five minutes or so into the beginning of class. Please give extra notice for your planned use.


Do not run above 18 V. Train will wreck!

(There is probably a 2 V margin up to 20 V, but a wreck causes serious damage.)

The switch on the battery car turns on the system. Switch off after use to conserve batteries. (The switch on the gun car should be left on.)

  1. Start with gun set to low power. Use flashlight to shoot while train is at rest to show that gun catches ball.
    (Allow gun capacitor time to recharge between shots. Usually one turn around the track is enough.)
  2. Run train slowly (14V). Let gun shoot on straight section of track.
  3. Repeat on curved section of track. Discuss inertial frames.
  4. Finale the tunnel. Set gun to high power. Run train at 18V. Set flashlight to discharge gun just before tunnel. Wear engineer's hat and blow whistle as train comes through tunnel.