Vacuum Cannon

If you're feeling pressure to do something fun, consider the vacuum cannon...

What it shows:

Air rushing into an evacuated tube pushes a ping-pong ball down the tube with such a force and speed that it blasts out the other end at 690 mph and easily passes through three or four cans of soda (they're empty), embedding itself into the fifth can.

CannonTarget

We learn two things. First, a modest pressure difference of 1 atmosphere, acting acting over a distance of a couple of meters, can impart a considerable amount of kinetic energy to a small object. Second, the impulse from a small light object traveling at a high speed can impart a surprising amount of damage to the object it strikes—a vivid lesson in kinematics.

One is reminded of the tragic 2003 space shuttle accident in which falling foam at liftoff caused the damage that doomed Columbia. Engineers at NASA thought lightweight foam could not harm the seemingly tough composite panels and privately predicted that the foam would bounce off harmlessly. Then researchers at the Southwest Research Institute in San Antonio shot a 1.7-pound piece of foam at a mock-up shuttle wing at 531 mph, roughly the speed of the chunk of foam that hit the Columbia wing about 81 seconds after liftoff. It was a jaw-dropping demonstration—people's intuitive sense of physics is sometimes way off.

How it works:

The tube is a clear, 8-ft-long, 1½" diameter PVC pipe.1 A ping-pong ball2 is placed inside and the ends are sealed off with a small piece of thin mylar3 held in place by PVC couplings that simply slip over the mylar. The tube is evacuated with a vacuum pump:

vacuum

Once evacuated, the pump is shut off and the mylar is punctured with an Exacto knife, causing the mylar to catastrophically rupture. If desired, an infra-red photogate at the other end may be used to measure the velocity of the ball as it blasts out. We have measured it to be 308±20 m/s. The ping-pong ball easily passes through three soda cans.

Setting it up:

C-clamp both the vacuum cannon and the soda can holder to the lecture bench.

References:

R. Peterson, B. Pulford, and K. Stein, "The Ping-Pong Cannon: A closer Look," TPT 43(1), 22-25 (January 2005)
E. Ayars and L. Buchholtz, "Analysis of the vacuum cannon," AJP 72, 961-963 (July 2004)

Footnotes:

The cannon was designed with the help of the collective wisdom and experience of the tap-L community...many thanks

  1. schedule 40; Grainger #1AAZ7. I.D.=1.610" O.D.=1.900"
  2. 40 mm nominal diameter, measured to be 1.562" or 39.7 mm in diameter and has a mass of 2.6 grams
  3. PrimaCare Foil Mylar Rescue Blanket (1.5 mil thick) sold as Emergency Mylar Thermal Blankets (pack of 10 for $6.90