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xocet

(3,870 posts)
Mon Apr 29, 2013, 06:23 PM Apr 2013

Supersonic Ping Pong Ball Cannon

Cub Scouts from Pack 119 shoot ping pong from PVC pipe cannon vaporizing watermelon
Beau Zimmer
6:20 PM, Apr 28, 2013


DES MOINES, Iowa- Have you ever dreamed of vaporizing a watermelon with a ping pong ball?

That was the mission for a group of 10 and 11 year-old Cub Scouts near Des Moines, Iowa on Thursday.

Cub Scout pack 119 built a 10 foot long supersonic ping pong ball cannon made out of PVC pipe. Turns out, the experiment was much more powerful than expected.

The ping pong ball was clocked at over 880 miles an hour. That's faster than the speed of sound!

...

http://www.wtsp.com/news/reporter/article/313795/79/WATCH-Cub-scouts-vaporize-watermelon-with-supersonic-ping-pong


One should note the contrast between the above and this:

Florida Teen Girl Charged With Felony After Science Experiment Goes Bad

http://www.democraticunderground.com/?com=view_post&forum=1002&pid=2772508
12 replies = new reply since forum marked as read
Highlight: NoneDon't highlight anything 5 newestHighlight 5 most recent replies

DreamGypsy

(2,252 posts)
1. Here's how it works...
Mon Apr 29, 2013, 06:57 PM
Apr 2013

...which is what every inquiring mind wants to know, so she/he can try the experiment at home.

Just in case you didn't follow the link to the full story from the Des Moines Register:

• A ball is placed in a 10-foot, 1.5-inch-diameter PVC pipe. The ends are covered with butcher paper using pipe fitters’ sealant. A pump sucks the air out of the tube, creating a vacuum.

• A heavier PVC pipe is attached to the tail end of the 10-foot tube. It’s pumped full of air, to a pressure of over 100 pounds per square inch.

• Eventually the pressure becomes too great, ripping through the butcher paper and pushing the ball through the vacuum at speeds faster than a passenger jet, the Cubs said.

Warpy

(110,913 posts)
2. Mythbusters will have fun with this one
Mon Apr 29, 2013, 07:10 PM
Apr 2013

It's better than their chicken cannon!

I guess destruction from guns of any type=good, no destruction from a harmless explosion=bad.

Stupid doublethink.

sir pball

(4,726 posts)
12. Sounds like a low-tech version of a light gas gun
Wed May 1, 2013, 11:47 AM
May 2013

Using pumped air instead of explosively-compressed hydrogen. I wonder if this would scale up to potatoes...or punkin' chunkin'

http://en.wikipedia.org/wiki/Light_gas_gun

ProdigalJunkMail

(12,017 posts)
3. at that range...
Mon Apr 29, 2013, 07:30 PM
Apr 2013

there didn't need to be a ping pong ball to achieve that destruction... the air blast alone would have done that. my guess is, the ping pong ball was already in shreds long before it came out of the end of the tube.

and about your 'contrast'... one was done under adult supervision in a non-school related activity, not on school property requiring no school sanction. the other... yeah, she blew up some shit on a school campus. now, is it stupid she is being charged? hell yes. however, one has absolutely nothing to do with the other. it's not apples and oranges. it's apples and clocks.

sP

 

TheMadMonk

(6,187 posts)
4. Actually that's the reason for evacuating the barrel.
Mon Apr 29, 2013, 09:20 PM
Apr 2013

It allows the PP ball to be accellerated to mach speeds WITHOUT air resistance.

ProdigalJunkMail

(12,017 posts)
6. the problem is the force of the air that provides the acceleration
Tue Apr 30, 2013, 08:01 AM
Apr 2013

air moving at 800+mph would at the very least crush the ball and more likely tear it open.

sP

 

TheMadMonk

(6,187 posts)
9. Your first post sugessted great disbelief.
Tue Apr 30, 2013, 01:01 PM
Apr 2013

Air blast at that range probably wouldn't rock that melon. Certainly wouldn't dent the skin, let alone blast it to pieces.

Do a quick google on the properties of a PP ball, and you'll learn that in theory the ball can withstand a crush pressure of just over 100 PSI and experimentally 50ish.

With a vacuum behind the ball, 100 PSI isn't going to do jack EXCEPT accelerate the ball until it bursts through the seal at the end of the barrel.

The first resistance is going to come when the ball hits atmosphere already moving at 880 odd MPH.

Watch the videos and be prepared to consume corvid.



This link is a slow mo PopSci video, showing that the ball remains largely intact until it hits it's target.

http://bcove.me/pbzl94cm

ProdigalJunkMail

(12,017 posts)
10. ok fine... i suggested the ball wouldn't survive the acceleration... so i was wrong
Tue Apr 30, 2013, 01:36 PM
Apr 2013

but a 800MPH column of air originating two feet from the watermelon would have destroyed it... ball or not.

but you go ahead and be 'right'... i am sure it is what you are good at.

sP

 

TheMadMonk

(6,187 posts)
11. Not an 800 MPH column of air. 100ish PSI air blast.
Tue Apr 30, 2013, 10:57 PM
Apr 2013

And if I read the diagrams correctly, supersonic air "pancakes" when it hits stationary air. (Something I'd already intuited.) It may form a vortex if the muzzle of the cannon is correctly shaped. Such a vortex would hit like an open hand slap. NOT going to damage the melon. Tory on Mythbusters took a similar blast in the guts from a SIX INCH cannon and oddly enough did not disintergrate.

People routinely (albeit a little dangerously) use an air hose at 100 PSI or more and ranges as little as an inch (or less), to dust themselves down in workshops all around the world. It's a little dangerous, (apart from the obvious risk of flying debris) because if the stream of air enters a cut in the skin, it again becomes contained and can lift the skin off an entire limb.

I am not (air quotes) 'right' I am CORRECT and YOU are now twice wrong on the same assertion, but still defending said false position with all the zeal of a Birther.

xocet

(3,870 posts)
5. "Hell yes!" is a great answer and is the important point. Thanks for checking out the other story.
Mon Apr 29, 2013, 09:50 PM
Apr 2013

n/t

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