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You often see action scenes in media, where a person is thrown so hard against a concrete wall, it breaks.

My question is if it's possible for a softer material to be accelerated to point where it would penetrate a harder material to a substantial amount. Does velocity even play a role in this? (Guns?) Or is it all material structure or hardness?

I guess I want to know which forces play a role at any given impact and what happens to both objects. Or does it all come down to material structure, flexibility and brittleness? I am really sorry if this is all over the place, I lack the physics education to phrase my question correctly.

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  • $\begingroup$ The answer to your question depends on what you mean by "high speed". High speeds on day-to-day human scales and speeds in excess of a few kilometers per second are quite different beasts. $\endgroup$ – dmckee Nov 25 '18 at 23:00
  • $\begingroup$ Well, I am aware of that. I guess for the movie related question it's always subsonic speeds but I guess the whole concept in general? How and when (at which velocity) it changes. If it's possible to accelerate a "softer", as in less dense inferior material structure etc, object to a speed where it damages a "harder" object to a substantial amount or does it always break, no matter the velocity? (e.g. would a copper projectile always bend/break/compress even when shot at super sonic speeds?) $\endgroup$ – Elias M Nov 25 '18 at 23:20
  • $\begingroup$ check out "the water jet channel" on youtube. Water goes through steel like butter, whether it is soft or hard water, too. $\endgroup$ – JEB Nov 25 '18 at 23:47
  • $\begingroup$ I guess that answers part of my question, but I am not talking about liquids! $\endgroup$ – Elias M Nov 26 '18 at 0:21
  • $\begingroup$ A softer material simply means that impact time is longer. this means that there is a lesser force of impact since change in momentum is equals to force times time. $\endgroup$ – QuIcKmAtHs Nov 26 '18 at 9:37
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I'm going to think that the soft material is so soft that it is something like a gas, I'm also gong to assume that the harder material is fixed in place, so it cannot move after the impact. If the softer material goes fast enough it will definitely cause some damage to the harder one, its kinetic energy must overcome the cohesive energy of the solid at the impact zone.

Note. I'm not 100% sure about this answer, I wish it was a comment instead. Can a mod or someone put it as a comment. Thx.

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Soft means that the material is more easily compressed. This would mean that the collision time of impact is greater. Since change in momentum is $Ft$, and $t$ increases, it would mean force decreases. So if you want to use a soft material to smash a wall, two things too take note are, of course the high velocity, and also the high coefficient of restitution. This makes a rubber ball highly ideal.

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  • $\begingroup$ This analysis is useful for speeds that are quite fast on the ordinary human scale, but fails at really high speeds. The issue is that molecule-molecule passing times get to be much smaller than the time-scale of inter-molecular relaxation in either body and the binding forces of either material pale before the dynamic close-approach forces. People in the high-speed collision business often say things like "everything is liquid".. $\endgroup$ – dmckee Nov 26 '18 at 15:18

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