Stokes pingpong cannon

PROFESSOR STOKES: What you are about to see actually happened.

We did not use any trick photography.

My name is Harold Stokes,

and I'm a professor of physics at Brigham Young University.

This is my physics class.

[APPLAUSE] Today,

I'm going to demonstrate the effects of pressure.

The atmosphere around us is always pressing in on us with pressure.

In this case, about 14 pounds per square inch.

Now, it doesn't seem like very much,

but that much pressure can produce huge effects,

as I'm about to show you.

What I have here is a long tube,

open at both ends for now and I'm going to put a ping pong ball,

in one end of this tube.

Then I'm going to seal off the end of the tube with a piece of plastic.

This is very similar to the shiny plastic that they use on those helium party balloons.

It's very thin but somewhat strong.

It's sealed off on this end.

I'm also going to seal off the other end.

Now, what I'm going to do is turn on a vacuum pump and pump all the air out of this tube.

Then I'm going to puncture the seal on this end of the tube.

The air will rush in and push the ping pong ball, down the tube,

and it will burst through the seal on the other end,

and then harmlessly hit the wall over there in that little hallway.

This is going to make a very loud bang.

Please cover your ears.

I will do it on three.

One, two, three.

Did you see the ping pong ball?

[BACKGROUND] You probably couldn't have seen it very well

because it came out of this tube going faster than 500 miles an hour.

That's about 70% the speed of sound,

and that's an acceleration of about 1,000 Gs.

Here's what's left of the ping pong ball.

It hit the wall with such force that it broke into pieces.

Shall we do it again? [APPLAUSE] This time,

I'm going to put a piece of plywood in its path.

This is eighth-inch plywood.

In order to catch the splinters,

we have this little device here.

Again on three. One, two, three.

[APPLAUSE]

Get a ping pong ball

sometime and see if you can throw it hard enough at a piece of plywood to do

this. One more time?

STUDENTS: Yes. [LAUGHTER] [APPLAUSE].

PROFESSOR STOKES: Good

to verify

for the class that I don't have any armor plating underneath this stuff here.

Go ahead and poke me in the stomach.

[LAUGHTER] I have only natural padding.

[LAUGHTER] Do I have a volunteer to fire it?

[LAUGHTER]

You. [LAUGHTER] You go ahead and do it on three.

STUDENT: Three, two one. [APPLAUSE]

PROFESSOR STOKES: I have a confession to make.

I cheated. Let me show you.

[LAUGHTER] Bet you never suspected.

[LAUGHTER] Actually, it really did hurt.

[LAUGHTER] I got to take a quick peek here to see what damage was done.

[APPLAUSE] I don't think I'll do that again.