Ball shot horizontally, one dropped vertically; both hit the ground at the same time.
What it shows:
The horizontal and vertical motions of a projectile are independent of each other. So two objects falling under the influence of gravity from the same height will reach the ground simultaneously, regardless of their horizontal velocities.
How it works:
The device, constructed by Nils Sorensen, consists of a spring loaded rod that shoots one of two billiard balls horizontally (pin-ball machine fashion). The second ball has a hole through it so that it slips over the other end of the rod. As the rod is released to fire ball #1, it slips out through ball #2 which then falls to the ground.
Setting it up:
The device clamps to a bench but protrudes over the edge giving the balls freedom of movement. A hard floor is vital here, because the best way to judge whether the balls strike the ground simultaneously is to listen for one or two cracks as the billiard balls land.