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Why a small wall falls apart on being hit by a bullet?

  1. Aug 5, 2015 #1
    When a bullet from a rifle hits a small wall, the wall falls apart. According to Newton's third law of motion, the wall should also exert an equal and opposite force as exerted by the bullet. The wall should be able to withstand the force exerted by the bullet on it. Why doesn't this happen?
     
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  3. Aug 5, 2015 #2

    sophiecentaur

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    Forces certainly come into this but what really count are Momentum and Energy.
    The speeding bullet will be brought to a halt in a very short time. Momentum is Mass times speed and it is one of the most fundamental things in the Universe (even!!!)
    The small mass times the high speed of the bullet gives a high momentum. There's a thing called Impulse, which is Force times time and Impulse is what causes a change in momentum. SO lots of momentum (taken from the bullet) and a very short time implies a large Force, which will be applied over the small area of the bullet nose (high pressure = high stress on the surface) as an Impulse is imparted to it by the target. But you can't be specific about the actual force involved because it totally depends upon how long the bullet takes to come to rest. That force will enable the bullet to break / penetrate the surface of the wall. The bullet can end up stationary (embedded). The wall will not actually 'go anywhere' because its momentum will be equal to that of the bullet and a big mass will have a small velocity (much much smaller than that of the bullet). In fact, it won't have any velocity because it's stuck to the ground (massive Earth). But the high Kinetic Energy of the bullet will set up shock waves in the brick, in all directions, which will cause fractures (all in a fraction of a second). The (brittle) wall may collapse or have a large hole in it. Bullets with different masses but the same amount of Kinetic Energy (same amount of powder in the cartridge) will have different effects and are suited to different targets. (This is turning disgusting!!!!! NRA please note.)

    Note, when someone is hit by a bullet, they will not be 'thrown backwards' because the Momentum (Mass times Velocity) results in a very small final person velocity (massive person). Hollywood got it wrong every time. The damage is done, as with the wall, by the energy being expended inside the person. The shock wave can cause death, even when the bullet only hits an outside bit. The entry hole (wall and person) may be quite small (bullet size) but as the bullet slows down (carrying bits of the target with it, no doubt) there will often be a very large exit hole where the bullet emerges much slower.

    Of course, the initial Force must be enough, on impact, to actually start all this off. A BB pellet will just bounce off but it has very little Energy (light and slow) and the energy you put into the spring is laughably low.
     
  4. Aug 5, 2015 #3

    CWatters

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    The wall has to deform in order to provide the force required by Newtons third law. Think of the wall as like a very stiff spring. A spring generates no force until it's compressed. There is a limit to how far an object can deform before it fails. Even a rubber band will eventually snap.
     
  5. Aug 5, 2015 #4
    The wall exerts an equal and opposite force on the bullet, not on itself. This force does not balance or cancel the force exerted by the bullet.
    On the wall there is only the force from the bullet and the force from the ground, which hold the bottom of the wall in place.
     
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