Two identical snow globes are smashed together, but only one breaks. Why?

[jordanmyoung]

Hello everyone,

I'm brand new to these forums and am a first-year physics student. I started taking the course to fill a requirement but have started to really enjoy the subject. I recently saw something on TV and wondered if anyone could explain it to me. A character was trying to prove a point by smashing two identical snow globes together, but the action caused only one of the globes to shatter. Was this just a special effect or can some principle in physics explain why this would happen?

Thanks for your help and hopefully I'm posting in the right forum.

Jordan

 

[Simon Bridge]

Welcome to PF;
Good question - it would be helpful if there was a link to the video online that we could watch. I'm guessing the program was the momentum deferred episode of Fringe?
http://fringepedia.net/wiki/Momentum_Deferred
No two objects can occupy the same space at the same time... and only one world would remain if they did collide. Nina demonstrates the concept by smashing two snowglobes together, one is destroyed, the other remains intact.

IRL: you can get this sort of thing because nothing is ever really identical.
Snow globes are usually pretty cheap blown glass so small irregularities in the thickness will make a difference.
And then - this is a TV show - the prop folk may well have weakened one of them.

 

[jordanmyoung]

Thanks, Simon. You're correct; it was that episode of Fringe.

My knowledge of physics isn't that advanced, obviously, but we just learned about Netwon's Third Law so I assumed that the force experienced by both snow globes would be equal. Are you saying that small differences between the two caused one to be more able to sustain that force than the other?

 

[K^2]

Simon, I agree on most of it, as far as cheaply made snow globes, but it feels like a more interesting problem if we assume they are more carefully crafted. Imagine I made two glass spheres that are almost entirely identical. Naturally, some imperfections will make one yield before the other. Question is, will this always relieve enough stress to prevent the other from shattering? Under some conditions? Never?

The reason I'm thinking this can be less than trivial is because if I consider a situation where I press the two spheres together with ever increasing force, rather than just smash them together, once one of the spheres yields, the other doesn't need to. So if material on both is sufficiently brittle and free of any but microscopic defects, I expect only one of the two spheres to break.

Dynamics of collision will certainly make a huge difference to how the spheres break, but will it change the outcome? Can shattering of one prevent shattering of the other?

 

[jordanmyoung]

Great points, K^2. In this example, it seems unlikely that the individual smashing the two together would be able to perfectly exert the force capable of breaking one but not the other, right? So assuming she overshot and exerted at least slightly too much force and assuming that the globes aren't very different, wouldn't we expect the threshold for shattering to be reached by both globes?

 

[A.T.]

I consider a situation where I press the two spheres together with ever increasing force, rather than just smash them together, once one of the spheres yields, the other doesn't need to.

Suddenly relieving stress can break an object too:
http://www.lmm.jussieu.fr/spaghetti/movies-3-exp.html

 

[Simon Bridge]

It does not have to be precise because collisions are not instant.
(The globes would start out deforming until one exceeded it's breaking point - the unbroken one springs back, which can break it of course.)
And even with very small differences, one has do break before the other. Of course, one could sort-of crack and then both break... which is why it is possible (even likely) that the used a deliberately weakened globe (or shot the scene several times).

I remember being surprised once smashing two bottles together and one remained intact.

I learned this as one of those chaos effect things. Very small differences can have a large effect on the outcome. To check we'd have to get a whole lot of snow-globes and do the experiment.

(Compare with bottles.)

 

[CWatters]

The moment one fails the pressure is relieved from the other. So a tiny difference for any reason is all it takes. Even if they were identical they may not be perfectly spherical so a slight difference in orientation might be enough.

 

[A.T.]

I remember being surprised once smashing two bottles together and one remained intact.

During egg tapping it is very rare that both eggs break. But eggs are of course far from identical. Usually there is an egg that beats all other eggs.

 

[Simon Bridge]

And then there is the sport of conkers.
http://en.wikipedia.org/wiki/Conkers
... also non-identical. Doesn't beat eggs, admittedly - you need a whisk for that.

Two solid crystal spheres striking will chip differently - showing how hard it is to get exactness.

I suppose one could rig a pendulum system with light-bulbs, to get them to hit as exactly as possible?

 

[jetwaterluffy]

No matter how identical they appear, one will always be harder to smash than the other. And once one starts yeilding, it becomes a lot less tough and will continue shattering due to being the easier path, whist the other one has no need to.