Why do fast objects cause more damage?

  1. A tiny pellet thrown by hand would never harm a glass window , but fire that pellet from a shotgun and it will shatter the glass into pieces , obviously the speed of the pellet made all the difference here .. how could this be explained on the actual atomic level (I understand the theoretical energy explanation).
     
  2. jcsd
  3. russ_watters

    Staff: Mentor

  4. Thanks for the welcome :)

    I know it's kinetic energy , but what is happening exactly at the microscopic level ? Do the atoms of the pellet become so hot at high speeds that they destroy the other object just through thermal insult ? or that the high speed gave the atoms of the pellet a much greater repellant force , repelling a massive number of the other object's electrons , effectively destroying it's atomic structure ?
     
  5. SteamKing

    SteamKing 9,932
    Staff Emeritus
    Science Advisor
    Homework Helper

    Well, if you threw a brick at the window, I don't think the glass breaks because the brick gets hot (which it doesn't).

    What is happening is that materials can absorb a finite amount of energy before losing cohesion. Glass can absorb a relatively low amount of energy. Steel, on the other hand, can absorb a relatively large amount of energy, per unit mass. If you throw a brick at a steel beam, the brick is likely to break instead of the steel.
     
  6. Ok , I understand that very well , but still that explanation doesn't really reveal what is happening microscopically .

    I am now perplexed whether the effect of high speed is like a heavy pressure on a small point phenomenon , or like a large mass acting on the glass .

    1-If like heavy pressure : the high speed pellet act as a unidirectional vector carrying great pressure .. it's atoms focused and packed as to provide the greatest repellant effect possible .. just like what happens when a sharp object penetrates flat surfaces , only this time there is no sharp object , the pellet gained the characteristics of the sharpness due to it's high speed .

    2-If like large mass : the high speed pellet act exactly like a pellet with a large mass , it's atoms repel large amount of glass atoms , just like what happens when a pellet with great mass hits the glass .. Only this time , the pellet has small mass , it gained the characteristics of a large mass , as a result of it's high speed which enabled it to repel larger than than the usual amount of glass atoms .
     
    Last edited: Feb 17, 2013
  7. Anyone?
     
  8. The glass breaks because you stretch it beyond its limit. http://en.wikipedia.org/wiki/Resilience The bullet has a high velocity that needs to drop to zero, for this the window has a certain distance before it is stretched to the breaking point. Accordingly the necessary acceleration and therefore the force is known, if the elastic modulus of the glass is too small it will break. There is also a bit of geometry involved so small things break glass more easily if they have the same kinetic energy and mass.

    The atomic mechanism for this is complicated. If one takes a naive approach and calculates what energy it takes to pull apart all the atomic bonds at once, the necessary force is insanely high. What happens instead is that in regions of high stress atomic displacements start propagating through the material, and depending on the atomic structure may come to a stop or create more displacements causing a chain reaction. This causes deformation and breaking.

    If you want to go one step lower the atoms of the bullet bump into the atoms of the glass. Their electron orbitals repel causing the atoms to push into the atoms further inside the material. A wave of momentum passes through the glass causing macroscopic deformations until a number of atoms are displaced enough that you would call their electron bonds broken.
     
  9. Excellent .

    So you would call that the high speed gave the atoms of the pellet more repelling power?
     
  10. Hey!
    To me, 0xDEADBEEF's answer was the best way to state it.
    The bullet has electrons which share the bullets momentum. So does the glass.
    The highest the bullet momentum, the highest its electrons' momentum and therefore the greatest the momentum transfer (and thus kinetic energy transfer) to the glass atoms.
     
  11. how exactly do the electrons share the momentum of the bullet ?

    Do you mean they gained more random motion (became hotter?) maybe went up to a higher orbit?or that the magnitude of their electric field became stronger?
     
  12. Well, actually no. They increase their momentum in the direction of the bullet.
    If you assume that instead of a bullet you had a group of particles, the center of mass of which increases velocity. Then, each individual particle's velocity increases.
     
  13. It is not about heat and not about "repelling power", whatever that is supposed to be. The breaking of glass can be modelled quite well using purely mechanics and breaking bonds. The reason more atoms are moved and more bonds break is simply because there is more kinetic energy in the bullet. If you throw a bowling ball into a field of 1000 bowling pins more pins will get knocked over if the ball is fast than if the ball is slow ceteris paribus. The ball doesn't need to be hot, or get "repelling powers". Faster bullets break more atomic bonds and displace more atoms than slower bullets.
     
  14. Brilliant ! That means the electrons move more in the direction of the bullet, than any other direction , thus providing a better chance to repel the glass's electrons , thus breaking the bonds afterwards .

    This shines a whole new level of light on the idea of acceleration and speed on the atomic level .

    Yeah I know , And that's why I am asking: why? by what microscopic mechanism was it able to do that ?

    Frankly I find the momentum explanation absolutely brilliant .
     
  15. FT = mv reflects the 'speed' of the pellet in your original post....

    And if you fire a ball of cotton, not much will happen...

    In a conductor, which can be pretty 'strong',that is a metal, it's pretty easy to displace certain elections...without permanent 'damage'....those in the [loosely bound] conduction band.....Just apply a voltage potential and a complete circuit and you can move those electrons.....you have to move some structural material, something in the lattice framework, to do 'damage'.
     
  16. So we could say that impact displaces more electrons than electricity .. isn't that right?
     
  17. The electrons are in their orbitals which affect each other. Its not like little point like things colliding with little point like things. The electrons cannot occupy the same orbitals, so the whole orbital structure gets squeezed, also affecting the nuclei. It is really more like chained atoms colliding. It is much more classical atom physics than you seem to imagine. Electronics will move more electrons than an impact, but only the weakly bound ones, and these get replaced by others. The electrons in the lower orbitals are really hard to remove, and changing the crystal structure depends on moving the much heavier nuclei. This is mediated by the electrons, but it is really the atoms that matter the most. Electrons will just hang around where their atom is.
     
  18. OxDEADBEEF if you take for example the covalent bond. There is a couple of electrons shared by two atoms to create a molecule.
    If you shoot an electron in them, it will transfer momentum and thus kinetic energy in them.
    This way they will not be able to keep the bond and it will break.
    (Very childish explanation but I think it does the trick)
     
  19. You still didn't delve into the vital question .. What is the difference between a slow atom and a fast atom that gives the fast one the ability to shatter another material?

    I know about kinetic energy and such ,However I am asking about the variable that changed microscopically at the particle/atom level as a result of high speed .. Is it electron momentum ? charge magnitude ? something else ?
     
  20. the answer to your question is momentum.
     
  21. Microscopic level and macroscopic level make no difference here, it is momentum or kinetic energy or speed, and it is applied to a small enough area to break enough bonds. But the whole bullet is pushing its front atoms into the material it is not a few electrons in front that have new powers now.
     
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