Ping Pong Ball Cannon

[Clancy Arnold]

WOW WOW. Gives a good lesson in simple air-pressure, the very thing that allows our models to fly on wings.

And yes that is one attention-getter for a class. When I instructed in Academics in the USAF, both Navigator and Pilot Training, Attention Getters were the "thing".

This is the best I ever saw, and I dreamed up some good ones. Nothing ever to compete with this!

Horrace Cain



I bet it would be a fun to attend this guy's class.
After this physics demonstration, this professor will have his classes undivided attention forever.


http://www.wimp.com/demonstratescannon/

Clancy

[dennis lipsett]

Somehow I have to be skeptical of a professor that shoots himself after seeing the force that the ping pong ball was capable of hitting him with. I sent it to my closest friend also with a doctorate  and told him to send it to his son who is defending his thesis next week and told him to tell him not to end up like that.

Dennis

[Tim Wescott]

Somehow I have to be skeptical of a professor that shoots himself after seeing the force that the ping pong ball was capable of hitting him with. I sent it to my closest friend also with a doctorate  and told him to send it to his son who is defending his thesis next week and told him to tell him not to end up like that.

Aren't you supposed to test that sort of thing on a graduate student first?

[Randy Cuberly]

My first year Chemistry teacher in High School, set up a demonstration of pressure and explasoves by using a large icylinder of glass (about 15 inches in diameter and about a foot and a half tall, with a lit bunsen burner inside.  He also had a pile of common wheat flour on the opposite side of the cylinder (still inside the cylinder) with a rubber tube running into the pile.
In come the unsuspecting students...after greeting us he placed a metal trash can lid on top of the cylinder then blew into the tube scattering the flour...the resulting explosion blew the trash can lid to the ceiling (a tall ceiling) and left a permanent mark there.
Talk about getting attention...several students actually ran from the room.
After things calmed down he explanined about powder and dust explosions and pressure...everybody listened intensely.
I majored in Chemistry for a long time after that.

Randy Cuberly

[david beazley]

Could one of you science majors explain to a lowly art major why the ping pong ball doesn't rupture inside the tube that is being evacuated?  Isn't it basically a balloon, granted with thicker walls?

[Randy Cuberly]

Could one of you science majors explain to a lowly art major why the ping pong ball doesn't rupture inside the tube that is being evacuated?  Isn't it basically a balloon, granted with thicker walls?

Well the ping pong ball is a sphere and that's a very strong pressure vessel because the pressure is evenly distributed through out the structure.  I'm not sure what the pressure is inside a ping pong ball but I suspect it's atmospheric which is the same approx. 14 PSI airpressure that was being evacuated in the tube.  Also understand that with the pump used and the time involved in pumping down the pressure inside the tube it was still positive (not that that is particularly important to the question), which means that the pressure inside the ping pong ball was only a maximum of approx 14psi which simply isn't enough to rupture the sphere.  I would guess it would stand many times that before rupturing.  The reason the ping pong ball accellerates so quickly is because it has very low mass a significant amount of area for the incoming air pressure to act on and virtually no drag except possibly skipping along the inside of the tube (remember there is very little air inside the tube).  The impact of the ping pong ball on an object like the plywood is high because of the velocity of the ball.  In fact I wouldn't be surprised if the ball shatters when it hits the sealing plastic on the end of the tube.  Single point impact on a sphere overcomes the even pressure inside the sphere and literally punctures it.  Actually the same thing happens to steel ball bearings when the elastic deformation of the metal is exceeded.  Of course they don't explode like the ping pong ball but they do take a permanent set and are no longer round.
That's about the best I can explain it without producing a structural analysis of the ping pong ball.  Not gonna do that.

Randy Cuberly

PS:  due to the low mass and high drag coefficient of the ball in the air it's going to slow down very rapidly once it leaves the barrel.  You could probably catch it about 15 feet away (a guess)

[david beazley]

Thanks for the explanation Randy, makes sense to this art major.  I figured the low mass was why the prof offered up his physique to the demo.