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Bullet vs Punch

  1. Apr 2, 2008 #1
    Hi all,

    One idle night a friend and I started an argument over the question:

    "How much energy is imparted on to a person if they
    a) were to be punched
    or b) getting hit by a bullet ?"

    The ultimate aim is to find out which imparts more energy, which has more force and also which has more power. We debated for a while but found we could not quite recall as much physics as we one were able to.

    Assumptions:
    - No friction
    - Recipient is an average weight male (~83 kg) wearing a bullet-proof vest
    - We assumed we were using a Glock 19 with 9x19mm Parabellum with velocity of 300m/s and bullet weight 9.5g FMJ
    - Not sure about the strength of the punch but we assume it is equivalent of a Boxer's full weight, full force, style punch

    Thanks
     
  2. jcsd
  3. Apr 2, 2008 #2

    Andy Resnick

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    The answer is fairly easy to calculate: the bullet has a kinetic energy of 855 J, while a punch has an energy of around 300 J:

    http://www.science.ca/askascientist/viewquestion.php?qID=821

    Asking which imparts more force is not a well-defined question, better to ask about the power. Make some estimates about stopping time and you'll figure it out.
     
  4. Apr 2, 2008 #3
    Hi,

    I'm the second person. I think a punch would impart more energy into the recipient.

    Perhaps I'm confusing myself when i say energy tho, as i assumed that if the recipient moved back more, they received more energy..?

    Also, i said that, since people shoot guns and sustain a small amount of recoil, and people get punched and sometimes get thrown backwards, that there has to be more energy in a punch.

    A bullet receives all its energy in one quick bang, and once its left the barrel, receives no extra energy, meaning it can only transfer as much energy into the recipient as it itself received from bing shot from the gun.

    But a punch has a certain amount of kinetic energy at the point of impact, but then continuous to receive energy through the full duration of contact time.

    would a suitable test not be holding a gun against an object, and firing it (obviously in the opposite direction) and seeing how far backwards it moves, then punching it?
     
  5. Apr 2, 2008 #4

    rcgldr

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    A lot of the energy from the bullet would be absorbed by the bullet proof vest. In spite of the fact that the bullet has more energy, the punch probably has more momentum, and it's slower rate of speed and higher force would "go through" the bullet proof vest easier, as the vest is designed to absorb a lot of energy from a bullet sized object.

    Still even with a vest, there's bound to be a bruise that no body punch would leave.
     
  6. Apr 2, 2008 #5
    This does not show that punch has more energy, but it shows that it has more momentum. Momentum equals p=mv and energy equals E=mv^2/2, so E=p*v/2. Obviously one object can have more energy and another more momentum if the second object is lighter, but much faster: this is exactly the case in bullet vs. punch comparison, since an arm is much heavier and much slower than a bullet.

    However the energy of a punch is difficult to determine, because all body parts can contribute to the strength of the punch, but they don't move with the same speed. An exact calculation should include integrating v^2*dm/2 over both bodies:

    Estart=Integral(v^2*dm)/2 over both bodies (just before the punch)
    Efinal=Integral(v^2*dm)/2 over both bodies (just after the punch)

    (If the centers of gravity move horizontaly, then Integral(h*dm) should also be included in energies)

    Work=Estart-Efinal

    But this work would still not tell us the exact effect of the punch, since the effect also depends on the area of the strike.
     
    Last edited: Apr 2, 2008
  7. Apr 2, 2008 #6

    Mech_Engineer

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    This may be of some use to you-

    http://en.wikipedia.org/wiki/Commotio_cordis

    Also useful is this website:
    http://www.science.ca/askascientist/viewquestion.php?qID=821

    Overall, I would say it takes a very strong professional athlete's blow to equal the energy in a standard 9x19mm round. Other studies calculate karate punches at around 150J, so there is obviously a large level of variation between estimates. Overall, it's the area over which the kinetic energy is dissipated that differentiates the two attacks.
     
  8. Apr 3, 2008 #7
    Area has no relevance, the bullet proof vest cancels out the possibility of the bullet passing through the recipient.

    All of the bullets energy is transferred into the recipient.

    Also i think alot of calculations are compleetly wrong, for example:

    "We can calculate the energy of the blow using the kinetic energy = 1/2 mv^2 calculation. The mass of the fist plus forearm is 2-4 kg, so the total energy is in the range 100-450 Joules, or 70-330 foot-lbs."

    That statement is just plain dumb as a person doesn't just make their arm move then go totally limp as they make contact. And they don't just stand still and only move their arm, they put as much of their bodyweight into it as they can.

    Even if they only managed to put 10Kg of weight behind the punch (which i think is minimal) the punch should already have more momentum

    and then theres the fact that punches do not decelerate as normal as the puncher keeps using his muscles to propel his fist.
     
  9. Apr 3, 2008 #8

    Doc Al

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    That's the classic "cut off your arm and throw it" model of punching. Pretty silly indeed! :rofl:
     
  10. Apr 3, 2008 #9
    But I think it is pretty accurate to say that the energy needed to propel that arm forward (whether it is from the shoulder, the entire arm + the back, the entire body, or an arm throwing robot) is going to be roughly equal to the energy needed to stop it(energy delivered to target).
     
  11. Apr 3, 2008 #10
    You mean 427.5 J.
    With a Beretta 98 G Elite II (cal 9X21, average bullet speed ~ 350 m/s) the energy would be around 490 J.
     
    Last edited: Apr 3, 2008
  12. Apr 3, 2008 #11

    Andy Resnick

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    Oops- forgot the 0.5 in the front. Thanks!
     
  13. Apr 3, 2008 #12
    Imagine this model: a man with a mass 80 kg runs with a speed 5 m/s, extends his arm straight and crashes into another man. The energy of the punch was 1000 J (assuming the other man did not receive any kinetic energy: let's say he stood next to the wall). In this case a calculation of the kinetic energy of the fist/arm would not tell us anything usefull.
     
  14. Apr 3, 2008 #13
    In that example, the entire body is moving forward, therefore it is probably adequate to take the body's mass into account.

    To extend your example, if it turned out that the man was thrown forward by a giant slingshot at 5 m/s, it would not be proper to include the mass of the slingshot in calculating the energy received by a brick wall when the man ran into it.

    That the concept of using the the mass of the arm+fist is essentially correct can be seen that experimental measurements give the same result as the calculations using that assumption.
     
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