A vacuum cannon is an interesting physics demonstration that shows how much atmospheric pressure there is at the earths surface, though we do not feel it because our bodies have the same amount of pressure both inside and out. By removing air pressure from inside of a length of pipe the balance with atmospheric pressure is removed, and if the seal is broken on one end air pressure will rush in quickly enough to throw a projectile with a good deal of force.
To learn how I made the vacuum pump used to operate this cannon, take a look at this video: http://youtu.be/UkPLu6mgdWA
The pump is very high quality, and pulls a vacuum much faster than most commercially available pumps, making it ideal for large tasks such as this cannon. Most other pumps would take many minutes to bring a chamber that size down below 25 in/Hg of vacuum.
I first learned of vacuum cannons thanks to this Instructable, and was interested enough to take the project on for myself: https://www.instructables.com/id/Vacuum-Cannon-drives-ping-pong-ball-at-supersonic-/
First Prize in the
Launch It! Contest