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It is usually said that you can minimize the damage caused by a car crash by increasing its duration. This way the impulse (F x dt) will be the same, but the F component wil decrease and hence the acceleration for the passengers. For example, by manufacturing the cars of steel that is deformable, you make them stick to each other and thus travel together during more time, instead of bouncing off and accelerating sharply. Logically, the opposite is also true: by chosing an elastic naterial, you would reduce collision time and increase the damage. But I wonder if you can obtain this result without changing the actors, without altering the display, just by reducing the interaction time.

For example, the bird pecks at the wood of the tree. Of course, thus the surface exposed to the force is reduced and this maximizes the presure and the impact of the force. But the bird also uses quick repeated movements for some reason, doesn't it?

I tend to think that the same rationale lies behind in both cases.

When the interaction time increases with a deformable car, it is because this material is like a coward army with little cohesion. If the soldiers (molecules) are hit and displaced, there is no courage (restoring force) making them strike back, so they are disbanded (potential energy is not re-converted into kinetic energy). Hence the non-bouncing off and the longer interaction time.

When the interaction time decreases out of the sheer will of the bird, the effect is that its beak, even if it faces a brave army, just hits a couple of soldiers and retreats before they can obtain assistance from their colleagues... This would be a sort of hit & run strategy, guerrillas war... reducing the attack time and maximizing the force?

Well, that is what I initially thought but have doubts...

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closed as primarily opinion-based by Gert, user108787, heather, John Rennie, AccidentalFourierTransform Nov 25 '16 at 16:45

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ It is not clear to me what you are asking. You seem to be just expressing your ideas and looking for approval. $\endgroup$ – sammy gerbil Nov 25 '16 at 0:22
  • $\begingroup$ Woodpeckers have extremely specialised mechanisms which allow them to peck the way they do without getting brain damage. Humans ... don't. $\endgroup$ – tfb Nov 25 '16 at 0:23
  • $\begingroup$ VTC as per sammy gerbil, no actual physics question. Sorry. $\endgroup$ – user108787 Nov 25 '16 at 0:29
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by manufacturing the cars of steel that is deformable, you make them stick to each other

It is not so much a "sticking together" effect, but rather a "pillow" effect. Even if they don't stick at all, the deformation still absorbs energy. This reduces the total kinetic energy.

Logically, the opposite is also true: by chosing an elastic naterial, you would reduce collision time and increase the damage.

"Elasticity" is not the opposite of "deformable". You can easily have a non elastic surface that doesn't deform. A stone for example. Those two are not opposites but rather in the same category: first a material elastically compresses/stretches and then it permanently deforms. Not either one or the other. These are called elastic deformation and plastic deformation, respectively.

Both the amounts of plastic deformation and of elastic deformation are positive factors increasing the duration of impact. Think of a deformable car and think of a trampoline; both reduce impact time and save your from injury.

When the interaction time increases with a deformable car, it is because this material is like a coward army with little cohesion. If the soldiers (molecules) are hit and displaced, there is no courage (restoring force) making them strike back, so they are disbanded (potential energy is not re-converted into kinetic energy). Hence the non-bouncing off and the shorter interaction time.

(I believe you mean "longer interaction time" in that last sentence here.) This analogy is fine so far, as far as I can see.

When the interaction time decreases out of sheer will of the bird, the effect is that its beak, even if it faces a brave army, just hits a couple of soldiers and retreats before they can obtain assistance from their colleagues.

This analogy is a little bit odd. I'm pretty sure (without being bird expert) that the quick head withdrawal happens because if simply bounces back. The backwards motion is not "out of sheer will" but simply the bounce. Then the impact is not independent of the motion afterwards, because the momentum change is larger which is a force applied in the wood. I do not believe the "retreating fast to reduce impact time" analogy is working here and I do not see the correlation with car collisions.

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  • $\begingroup$ Steeven@. As to last paragraph, I understand and agree $\endgroup$ – Sierra Nov 25 '16 at 7:58
  • $\begingroup$ Steeven@. FIRST PARAGRAPH: ok, I agree that no literal “sticking together” is necessary, what is key is deformation, although you may agree with me that deformation means less transfer of KE and hence more or less motion in unison of the two bodies after the crash. $\endgroup$ – Sierra Nov 25 '16 at 14:35
  • $\begingroup$ PARAGRAPHS 2 and & 3: there are 2 understandings of "elasticity”. Mine: after the collision, there's zero permanent deformation = there will have been, at microscopic level, compressing/stretching but restoring forces have re-stablished equilibrium = all PE has re-converted into KE. That is the meaning when we refer to a fully “elastic” or the ”elasticity” modulus of a material (although actually“rigidity” looks like a better word). The other is yours = “temporary and macroscopic” deformation, isn’t it? I missed this concept, a useful one = a property that enlarges interaction time. $\endgroup$ – Sierra Nov 25 '16 at 14:46
  • $\begingroup$ PARAGRAPH 4: you are right, I will edit. PARAGRAPH 5: I realize that you are right, the beak bounces back. Still there may be some sort of "giving" in the bird behavior, like when you smash a nail into the wall and quickly release the grip over the hammer, thus reducing tension. Like when a boxer relaxes the neck upon receiving a punch. So the bird behavior might be an example of right the opposite of what I was saying, an example of increasing instead of reducing collision time, so as to reduce the damage. Do you agree? $\endgroup$ – Sierra Nov 25 '16 at 14:47
  • $\begingroup$ @Sierra Yes, by "elasticity" I mean what the elastic modulus refers to. But there's no difference from the microscopic to the macroscopic view on this - for both cases, the elastic effect (compressing and returning back to original size) increases the impact duration. Think of when the object hits and when it leaves again: the more elastic (microscopically or macroscopically), the more contact time, because the compression and returning to original size takes longer time. The reason a trampoline is working is because it at the molecular scale is more elastic than, say, granite. $\endgroup$ – Steeven Nov 25 '16 at 19:57

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