Monday, February 23, 2015


Some readers may be familiar with curious and engaging air vortex launchers such as the Airzooka or Zero Blaster.

But when a physics demo is as groovy as this, you can count of someone to figuratively put it on steroids.

I was looking for a video of a vortex cannon blowing out a candle and found this promotional of the Candle Cannon. The suspense builds as the distance is steadily increased.

Erbert and Gerbert's Candle Cannon

You can see a slight visual distortion as the vortex reaches the cannon - a nice way to show that the compression of air is denser than the air around it as it refracts the light.

I'm thinking of using the video time stamps to find the speed of the traveling air compression with my students.  It looks like it takes 9 seconds from the sound of the diaphragm moving to when the candles get blown out for the last trial of 180 feet. That means the large "puff" of air travels 180 feet in 9 seconds or about 20 feet per second. Comparing that to the trial at 120 feet which took about 6 seconds that speed seems fairly consistent.

"But Mrs. Barnett! Mrs. Barnett! The sound takes some time to get to us (camera)!" Ahh if only they would all think that deeply.

If we assume 340 m/s for the speed of sound, or 1115 ft/ sec, its not offsetting our times that much. And just mentioning that fact that should help to point out that while the vortex cannon is used to model the compression of a sound wave it is not in fact a sound wave that reaches you. It's traveling at ~6% the speed of a sound wave after all. My kids love the air cannon in my classroom so I think they will enjoy this!

I have watched the video several times now and I found myself wondering what would happen if they started the cake much closer .... at some point it should take some of that frosting off the cake, right?

UPDATE: The Candle Cannon has its own website:, of course.

Monday, February 09, 2015

Conservation of momentum: not always your friend

Make of this what you will, gentle readers. But don't doubt the physics! We've got combustion, fluid flow, conservation of momentum / Newton's 3rd law, and an inclined plane.

Firefighters try to extinguish a car fire when suddenly...

And remember: if you're going to set your car alight on the top of a hill, have the courtesy to set the parking brake!