Step 1: How It Works
The "cannon" is basically a 6 foot length of 1 1 /2" PVC pipe. A ping pong ball is placed at the near end, and both ends are sealed with thin plastic or aluminum foil. The air is then evacuated from the pipe with a vacuum pump. When the seal at the near end is ruptured, air at atmospheric pressure rushes into the pipe and propels the ball through the seal at the far end.
The key concept is that the ball travels through a vacuum in the pipe, therefore there is no air resistance. With full atmospheric pressure on one side of the ball and no resistance on the other, the ball initially accelerates at about 5000 g's.
The photo shows the setup.