# What's driving the bucket up?

Just saw this cool video from Plymouth University, which I actually found through Matthen's blog.

They fill a plastic bottle with liquid nitrogen, screw the cap on, drop it in a bucket full of warm water, cover it with ping pong balls, and when the heated, expanding gas bursts the bottle, the balls go jumping all over the place...

If you scroll the video to 3:50, you can see that the bucket jumps in the air, seemingly at the same time as the ping pong balls. Of course the force of the explosion is not pushing it up, but down. So what exactly is making it rise over 1 m off the ground?

I can only think of two possible explanations:

• Elastic recoil from the material of the bucket. This seems pretty unlikely to me, as in my experience plastic buckets don't bounce much.
• Pressure difference: the explosion drives everything out of the bucket, leaving a partial vacuum behind, so the atmospheric pressure outside the bucket pushes it up before air rushes back in to equalize the pressure. This seems more likely, but I would had thought that there would have been a longer delay.

Am I leaving any other possible explanation? Anybody willing to tackle a back of the envelope calculation of the height or delay for either of these?

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Related experiment: physics.stackexchange.com/q/29523/2451 – Qmechanic Oct 2 '12 at 21:16
Video link is dead, here is a gif: cdn.fullpunch.com/wp-content/uploads/2014/04/238.gif – David Starkey May 14 '14 at 16:05

I'm willing to bet that it's simply the force of the explosion pushing on the bottom of the bucket, which pushes against the floor and makes the bucket bounce up into the air.

I doubt that it has to do with a vacuum forming inside the bucket. You're correct in thinking that we should expect more of a "delay" for the bucket to jump in this case.

And in any case, I don't believe that the explosion actually creates a vacuum in the bucket. It's simply the liquid nitrogen expanding into a gaseous state. The pressure in the bucket should never drop below 1 atmosphere. (it's not the same as an exothermic explosion, which does create negative pressure)

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Hadn't thought of how this "explosion" really isn't an explosion after all... Looking at the cloud that forms, it is pretty clear that there is no inward reflux of any kind. – Jaime Sep 27 '12 at 21:02

I'm with Dmitry on this one. Note that before the bottle explodes there are some ping pong balls lying on the floor next to the bin, and when the bottle explodes those ping pong balls fly up (though not as much as the bin).

The explosion deforms the floor below the bin, and when the floor rebounds it flings the bin into the air. The spilled ping pong balls alongside the bin also get thrown up, but because they are a little way away from the bin the deformation of the floor is less and they get thrown up less.

I wish we had done experiments like that at school :-)

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Deforms the floor, really? – LarsH Sep 27 '12 at 18:26
Yes, I'm willing to bet that if you put a ping pong ball on that floor then dropped a heavy weight you'd see the ball bounce up as the floor bent down then rebounded under the impact of the weight. – John Rennie Sep 27 '12 at 18:31
In fact I have just tried the experiment in my living room and I can confirm that the ping pong ball does bounce up when I jump off a chair onto the floor. Perhaps I should post the experiment on YouTube as proof :-) – John Rennie Sep 27 '12 at 18:35

@Jaime you've got it right. Any shock front (compression wave) has a Rarefication wave following it. But keep in mind that the downward directed shock will also reflect and end up going through the top of the bucket. But even that isn't necessary here.

In either case you have a slug of water being forcibly ejected. This mass of water will form a partial vacuum behind it.

While it is possible that the first (the upper directed shock) will carry water along with it, especially if the bottle was submerged. It's more likely that the downward directed shock forces water rapidly downward, which reflects off the bottom in a semi-uniform front which then exits out the top.

An analogy would be slomo shots of when a water droplet hits water. You always get a reflected bit.

And yes, this is a real explosion

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