Liquid Nitrogen + 1500 Ping Pong Balls — Parallax Forums

# Liquid Nitrogen + 1500 Ping Pong Balls

Posts: 20,256
edited 2012-09-27 20:38
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• Posts: 23,514
edited 2012-09-26 10:52
Quite an impressive demo! But here's a puzzle: can anyone explain why the trashcan leaps off the floor? What happened to equal and opposite reaction, which "should" have driven the trashcan the other direction, into the floor.

-Phil
• Posts: 20,256
edited 2012-09-26 11:11
The same reason a helium balloon in your car (tied off and free-floating) moves forward when you accelerate and backwards when you brake.

Gremlins.
• Posts: 4,051
edited 2012-09-26 11:12
Quite an impressive demo! But here's a puzzle: can anyone explain why the trashcan leaps off the floor? What happened to equal and opposite reaction, which "should" have driven the trashcan the other direction, into the floor.

Didn't think about that at first, but it is a good question. My guess is that the recoil did occur, but the trash is in contact with the floor, so that wouldn't be visible. The floor looks like it is wooden, so it flexed downwards and then sprung back upwards. The trashcan has less momentum than before as the ping pong balls are gone, so the kick it gets from the floor causes it to spring into the air.

If the trashcan was sideways the recoil would only move it backwards. It's the floor contact that is key.
• Posts: 726
edited 2012-09-26 11:17
Cause the upwards rushing of the expanding gas creates a vaccum behind it that sucks the lightweight trash can up.
• Posts: 20,256
edited 2012-09-26 11:18
The bottom of the trash can acted like a diaphragm (piston) and flexed outward (down) from the blast, while the PP balls were not attached and free to fly upwards, thus creating an imbalance of force and a singularity which will ultimately destroy the universe.

Nice knowing y'all.
• Posts: 5,045
edited 2012-09-26 11:20
Quite an impressive demo! But here's a puzzle: can anyone explain why the trashcan leaps off the floor? What happened to equal and opposite reaction, which "should" have driven the trashcan the other direction, into the floor.

-Phil

My guess is that the flexible bottom bowed out, pushing against the floor and propelling the trashcan up.
• Posts: 5,045
edited 2012-09-26 11:22
jdolecki wrote: »
Cause the upwards rushing of the expanding gas creates a vaccum behind it that sucks the lightweight trash can up.

But if that were the case than a cannon would recoil forward, not backwards like they do.

• Posts: 6,347
edited 2012-09-26 11:40
What happen's if you drop a rubber ball on a hard surface? It bounces. The bottle of liquid nitrogen was floating on top of the water. When it exploded it sent the ping pong balls up, and the water and the bottom of the trash can down. After a small fraction of a second the water and trash can could no longer go down, so it bounced up.
• Posts: 1,716
edited 2012-09-26 11:40
Mr. Moose would have liked this, Captain Kangaroo not so much.

C.W.
• Posts: 6,506
edited 2012-09-26 12:02
erco wrote: »
The bottom of the trash can acted like a diaphragm (piston) and flexed outward (down) from the blast, while the PP balls were not attached and free to fly upwards, thus creating an imbalance of force and a singularity which will ultimately destroy the universe.

Nice knowing y'all.

Good thing you're working on that Westminster Flame Ball clock!!!
• Posts: 1,915
edited 2012-09-26 13:01
Quite an impressive demo! But here's a puzzle: can anyone explain why the trashcan leaps off the floor? What happened to equal and opposite reaction, which "should" have driven the trashcan the other direction, into the floor.

-Phil
I think the bin was sitting on a conveyor?
• Posts: 3,347
edited 2012-09-26 15:25
I put dry ice in a can of paint and hammered on the lid, then got sent home for painting possibly the best thing ever in the school parking lot. Years later I discovered MREs an NVGs. 2 liter soda bottles, an MRE heater, all the Tabasco sauce we could find add water cap the lid and slip it in some boots tent.

Phil - Newton's third law of motion.

Back in the day there was this place called Cryodyne and they used to give us for free liquid nitrogen! They would fill a 7-Eleven styrofoam cooler 1/3 of the way and poke some holes in top. What a fun ride home that was.
• Posts: 20,256
edited 2012-09-26 15:40
skylight wrote: »
I think the bin was sitting on a conveyor?

You're an absolute fiend for poking that hornet's nest, and I thank you sir!
• Posts: 23,514
edited 2012-09-26 16:35
I'm going with the trashcan's deformable bottom acting as a piston against the floor. If it were just a case of the floor flexing and restoring kinetic energy to the can trampoline style, there would be more of a delay before the can leaps up.

-Phil
• Posts: 870
edited 2012-09-26 21:28
I'm going with the trashcan's deformable bottom acting as a piston against the floor. If it were just a case of the floor flexing and restoring kinetic energy to the can trampoline style, there would be more of a delay before the can leaps up.

-Phil

I think it's the floor flexing. You can even see a few of the spilled balls jump off the floor at the same time as the trash can. (actually I think I can see the floor flex a bit in the slow motion replay too) As for the lack of delay, I'd expect the floor to be very stiff. It could easily compress and re-bound in 50-100 mili-seconds. That would look "instant" to human eyes.

Lawson
• Posts: 6,559
edited 2012-09-26 22:59
I think there is something to this as a possible answer...

...look at some of the other ping pong balls around the trashcan between frames 1 and 2... they go up as well at the same moment the trashcan does, indicating some reaction of the floor.

The explosion drives the trashcan into the floor by equal and opposite reactions from the explosion. The floor does flex as a response to the trashcan. The recoil of the floor causes the trashcan and some of the immediate surrounding ping pong balls to go up in synchronization as well.

As far as Newton's cradle, if one ball is raised and allowed to drop on a stationary ball, then on the opposite side only one ball will bounce out. Ok, we all see that, but if the stationary ball, happens to be a wall, then the ball that is dropped will 'bounce' against the wall (at twice the frequency).... rotate this 90 deg where the wall becomes the floor and apply the trashcan as if it were a ball. As far as the floor is concerned, there was no explosion, and the trashcan was dropped from some imaginary height.

As far as delay in time... based on the video assuming about 30 frames per second, there is at least one frame of delay from when the explosion occurs and the trashcan lifts off of the ground.
• Posts: 23,514
edited 2012-09-26 23:09
'Not buyin' it, Beau; sorry. Granted, the balls on the floor do get propelled upward from some vibration. But if the floor flexed enough to bounce the trashcan to such heights, those balls would've been launched nearly to the ceiling. As it is, though, they barely bounce. I'm convinced that the lion's share of the can's launch energy comes from the piston action of a deflected bottom.

-Phil
• Posts: 1,982
edited 2012-09-26 23:43
Skylight What are you thinking??????

I think the bin was sitting on a conveyor?
++++++++++++++++++++++++++++
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++++++++++++++++++++++++++++

AAAAAAAAAARRRRRRRGGGGGGGGGHHHHHHH

Nnnnnnnooooooo!!!!!!!!! Make it stop>>>>make it stop!!!!!!!!!!!
• Posts: 1,982
edited 2012-09-26 23:54
'Not buyin' it, Beau; sorry. Granted, the balls on the floor do get propelled upward from some vibration. But if the floor flexed enough to bounce the trashcan to such heights, those balls would've been launched nearly to the ceiling. As it is, though, they barely bounce. I'm convinced that the lion's share of the can's launch energy comes from the piston action of a deflected bottom.

-Phil

I would have to put my guess on the impact of all the ping pong balls being propelled upward and outward faster than they can exit the barrel. That force is translated into an upward force on the barrel wall by the ping pong balls in contact with it. The balls have all this upward and outward (maybe someone could figure the vector model of this motion) momentum after the explosion and it has to go somewhere. As to the surrounding balls on the floor jumping, my guess is that would be caused by the shockwave radiating outward through the floor. Similar to banging a surface with a hammer and seeing the nails laying on it jump even though the flex is minimal.
• Posts: 1,915
edited 2012-09-27 00:41
And I only counted 1499 balls!
• Posts: 4,051
edited 2012-09-27 05:57
'Not buyin' it, Beau; sorry. Granted, the balls on the floor do get propelled upward from some vibration. But if the floor flexed enough to bounce the trashcan to such heights, those balls would've been launched nearly to the ceiling. As it is, though, they barely bounce. I'm convinced that the lion's share of the can's launch energy comes from the piston action of a deflected bottom.

There's only one way to settle this. We need to conduct two additional experiments to prove or disprove our hypothesis.

First we confirm that floor contact in the direction of motion is necessary for the reverse kickback. To do this we conduct the experiment with either the can on its side or suspended off the floor.

Second, we need to figure out if it is the piston effect or a variation on Newton's cradle. We'll use a trash can with a rigid bottom that is flush with the floor to eliminate the piston effect.

I nominate Erco to conduct these experiments as his experience with flaming Boe-bots indicates he likes danger.
• Posts: 1,716
edited 2012-09-27 06:21
So if we attach a sensor to one of the ping pong balls and tie that to a system on an elevator that instantaniously matches it upward velocity, will it ever escape the trash can?

C.W.
• Posts: 6,559
edited 2012-09-27 06:43
I'm going to stick with the hypothesis that it's a variation of Newtons cradle in disguise. The reference to a hammer striking a table and parts on the table jumping up... the hammer is also 'bouncing' from the table. Most of the energy will be returned at the point of impact and as you progressively go outward (law of squares?) the energy diminishes quickly, but this also depends on surface characteristics of the table/floor ... again a variation of Newtons cradle.

• Posts: 6,347
edited 2012-09-27 07:06
Conservation of momentum. Conservation of energy. Elastic collisions. Google it.
• Posts: 20,256
edited 2012-09-27 07:10
I'm sticking with Conservation of Elastic Gremlins on Treadmill with Coriolis.

• Posts: 23,514
edited 2012-09-27 07:16
Dave Hein wrote:
Conservation of momentum. Conservation of energy. Elastic collisions. Google it.
Both leading explanations take these into consideration, although the degree of elasticity can only be conjectured. So that doesn't help to pick between the two. I would venture to say that a polyethylene garbage can containing five gallons of warm water does not come close to resembling a steel ball-bearing as far as elastic collisions are concerned. Think about how the DOT protects overpass piers on the highway: that's right, large poly cans filled with water to provide inelastic energy absorption.

-Phil
• Posts: 6,347
edited 2012-09-27 08:22
"Both leading explanations"? Are you referring to the piston versus Newtons cradle? Of course you need both effects to explain why the trash can goes up. The trash can must have had downward momentum so that it could experience a collision with the floor, which would result in upward momentum. If the trash can was an inch above the floor when the exposion happened it clearly would have been accelerated toward the floor, and it would have bounced up when it hit the floor. In this case, it is more likely that the trash can deformed slightly when the explosion happened, which resulted in momentum toward the floor. The deformation was probably a combination of the slightly raised bottom and the angled sides. This caused the center of mass of the can to travel downward for a very short period of time.
• Posts: 23,514
edited 2012-09-27 08:56
For those favoring the Newton's cradle explanation, consider the following gedanken experiment. Instead of an explosion causing the downward acceleration of the can, drop it from a height of, say, five feet instead. Do you think the can will rebound? I would guess that, with its water content, it would behave more like a lead-shot-filled dead-blow hammer, with most of the kinetic energy being absorbed by the sloshing of the water and deformation of the can.

The difference with the explosion and the can sitting on the floor is that part of the can (i.e. the center of the flexible bottom) accelerates downward, pushing off against the floor. So it's more like a standing high-jump than an elastic rebound.

-Phil
• Posts: 6,347
edited 2012-09-27 09:07
It's possible that the explosion caused the trash can to act as a spring. It would have deformed downward slightly, and then sprung back up.
• Posts: 6,559
edited 2012-09-27 10:29
I still agree to disagree ... the trashcan is basically a rocket aimed down in this example. The thrust from the 'rocket' evacuates all of the balls, but at the same time exhibits the same force down on the ground. Now, since the fuel for this particular rocket is spent very quickly, the downward force is suddenly removed, the floor responds to this removal of force, similar to how a coil of wire does with a collapsing magnetic field when you remove the power. I'm not denying that there is some deformation of the trashcan that acts like a spring to store the energy, this absolutely happens, but it is also happening with the floor.