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Author Topic: Why does spaghetti break into 3 pieces?  (Read 3756 times)

Offline chris

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Why does spaghetti break into 3 pieces?
« on: 05/09/2014 08:59:07 »
I was speaking with a friend from the engineering giant Rolls-Royce the other day; he told me that at one of their recent interviews for new recruits they were asking candidates to explain why spaghetti break into 3 pieces rather than 2 when you hold each end and snap it.

So what's the correct answer?

Chris


 

Offline chris

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Re: Why does spaghetti break into 3 pieces?
« Reply #1 on: 13/09/2014 11:07:58 »
In order to ensure that this question does not go unanswered, I've checked the answer; what I suggested in a recent episode of the Ask! The Naked Scientists podcast is correct and also agrees with the kitchen science experiment we did previously (I'm relieved to see!).

My suggestion was as follows:

When a bending force is applied, the spaghetti deforms elastically, storing energy in the strand. When it breaks, initially in one place where there is a weak spot in the structure and usually not on the apex of the bend, the elastically-stored energy is released, setting up vibrations in the daughter fragments. These propagate along the still-bent fragments causing them to deform even more and causing the spaghetti to fail in at least one additional position.
 

Offline yor_on

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Re: Why does spaghetti break into 3 pieces?
« Reply #2 on: 22/09/2014 20:47:18 »
yep, seems possible Chris. A perfectly formed, evenly distributed density- spaghetti, without hidden 'faults', should then only break into two, right? At last a simple experiment )
 

Offline alancalverd

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Re: Why does spaghetti break into 3 pieces?
« Reply #3 on: 22/09/2014 23:23:19 »
I haven't tackled the spaghetti question yet, but I recall launching gliders with a tow truck and 600m of steel wire. If the truck accelerated too quickly from slack, the cable would break with a very satisfying bang, and always into 3 pieces.  But if you had established a constant tension and the glider was climbing smoothly, a little extra enthusiasm from the pilot could cause the cable to break in just one place.   
 

Offline yor_on

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Re: Why does spaghetti break into 3 pieces?
« Reply #4 on: 23/09/2014 00:01:10 »
Don't think I can afford that experimental setup Alan, I'll try it with a packet of spaghetti first.
 

Offline chris

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Re: Why does spaghetti break into 3 pieces?
« Reply #5 on: 25/09/2014 08:03:22 »
Don't think I can afford that experimental setup Alan, I'll try it with a packet of spaghetti first.
I haven't tackled the spaghetti question yet, but I recall launching gliders with a tow truck and 600m of steel wire. If the truck accelerated too quickly from slack, the cable would break with a very satisfying bang, and always into 3 pieces.  But if you had established a constant tension and the glider was climbing smoothly, a little extra enthusiasm from the pilot could cause the cable to break in just one place.   

Good observation, although I'm surprised to hear that the cables were that easy to snap - is this a weight thing? i.e. by necessity a less-than-ideal cable needs to be used to keep the weight down, sacrificing strength and the expense of mass?

C
 

Offline alancalverd

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Re: Why does spaghetti break into 3 pieces?
« Reply #6 on: 25/09/2014 18:02:38 »
You can't put more than about 1000 kg tension on a glider tow hook without bending the fuselage or breaking the wings, so we use a weak link at the top of the cable anyway. A 2-ton cable is thus entirely adequate when new, but it gradually gets abraded and weakened by kinks until it is discarded because it keeps breaking before the weak link gives way.
 

Offline PmbPhy

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Re: Why does spaghetti break into 3 pieces?
« Reply #7 on: 26/09/2014 02:10:37 »
I was speaking with a friend from the engineering giant Rolls-Royce the other day; he told me that at one of their recent interviews for new recruits they were asking candidates to explain why spaghetti break into 3 pieces rather than 2 when you hold each end and snap it.

So what's the correct answer?

Chris
There was special on TV about Richard Feynman.  For fun he tried to figuring it out but failed.
 

Offline RD

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Re: Why does spaghetti break into 3 pieces?
« Reply #8 on: 26/09/2014 06:33:40 »
Not necessarily 3 , just rarely 2 pieces ...




Quote from: Basile Audoly and Sebastien Neukirch
... Contrary to the intuition that removing a loading decreases stresses and so cannot induce failure by itself , we have shown that rods can break just because they are released. When a bent rod reaches its limit curvature and breaks at a first point, a burst of flexural waves is sent through the newly formed fragments, which locally further increase the curvature. The limit curvature is therefore exceeded again at later time, allowing a cascading failure mechanism to take place.
http://www.lmm.jussieu.fr/spaghetti/audoly_neukirch_fragmentation.pdf

« Last Edit: 26/09/2014 06:52:10 by RD »
 

Offline chris

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Re: Why does spaghetti break into 3 pieces?
« Reply #9 on: 27/09/2014 20:26:55 »
Thanks, RD, for the lovely video sequence. The quote puts it very well too.
 

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Re: Why does spaghetti break into 3 pieces?
« Reply #9 on: 27/09/2014 20:26:55 »

 

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