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This video shows what I mean.

Basically they shoot a bullet from a huge gun at an iPhone. What I don't get is why does the phone move only so little after being shot? As far as I can tell it isn't glued to the surface or something like that. Why isn't it blown to pieces? I imagine it has something to do with the speed of the projectile, since throwing a stone at it would certainly cause it to fly away with cracked screen.

Note: feel free to update the tags, I'm not really good at physics and have no idea how to properly tag this. Thanks.

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My understanding is that at the speeds involved the shear forces inside the material are so tremendous that the target no longer behaves rigidly, but more like a liquid, which you can see in slow motion videos of bullets hitting metallic targets. Because the bullet is moving so fast, the "information" that the bullet has hit the target travels faster than sound, i.e., a shockwave, giving rise to these enormous forces. Once the shear strength of the material is overcome, the friction/drag between the bullet and the target is diminished, so that the majority of the bullet's momentum makes it out the other end. If you want to send your iPhone flying, equip it with a bullet-resist vest, which can handle the forces in question.

Think of hitting an apple with a cleaver. If you do it slow enough, you are not providing enough shear force for (some of the) the cell walls to tear, so the apple stays in tact, and moves with the cleaver. If you do it fast enough, you end up with two halves that barely move.

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  • $\begingroup$ Is another way to say this, that since frictional force is directly proportional to the normal force and independent of speed, the faster the bullet passes through, the less work will be done on the iphone? $\endgroup$ – Niobius Jan 20 '14 at 21:40
  • $\begingroup$ @Niobius Not really. The precondition for the physics 101 description of friction are decidedly not met in a case like this. $\endgroup$ – dmckee --- ex-moderator kitten Jan 21 '14 at 2:14
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The phone's materials are too weak to cause much deformation of the bullet. The phone itself is designed to dampen and distribute the force of small impacts (to avoid internal damage). So once the phone's materials reach their elastic limit, the remaining force is then concentrated in a small area, which causes most of the phone at the point of impact to shatter and disperse.

This detritus ends up absorbing much of the transferred kinetic energy (as can be seen when it explodes out the rear at high speed) which means less of this energy is being transferred to the entire object. Also note how much the back of the phone becomes deformed; while the screen and internal components easily shatter, the metal back distributes some of the bullet's energy tangentially from the point of impact. Finally, the fact that the bullet is traveling faster than the speed of sound will cause the air in front to act as a sort of "buffer" prior to and during the impact. This decreases the amount of physical contact between the phone and bullet, lowering the friction between the two objects, thus again decreasing the energy transferred into the phone. These events all infer that a good deal of energy is being expended on forces that don't serve to displace the phone's remaining mass.

The bullet and the phone also have different masses. While the phone weighs roughly 112 grams, a 50cal sniper round might weigh between 39-48 grams depending on its type. Additionally, the stand the phone is attached to weighs an unknown amount. So the phone's much larger inertia needs to be overcome by a bullet whose forces are being largely redirected and dissipated.

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