4 ms·

The killer is not the momentum, but the impulse. Let's do some simple kinematics: The M82 has a barrel length of 20 inches (51 cm, .051 meters) and fires a 45

The killer is not the momentum, but the impulse. Let's do some simple kinematics: The M82 has a barrel length of 20 inches (51 cm, .051 meters) and fires a 45 gram bullet that, at the end of that time, is going 900 m/s. Meaning the bullet goes from having zero momentum to 41 Newton-seconds in 1.13 milliseconds. Meaning the shooter feels over that 1.13 millisecond period of ~36,000 Newtons of force on his shoulder. The person getting shot has to make that bullet go from 900 m/s to 0, but he has to do it over say 2 inches of distance or his organs start getting smushed. Meaning he's experiencing, over that 110 microsecond period, ~360,000 Netwons of force. The exact same momentum, but in the shooter's case it's like getting punched and in the shootee's case it's like getting your torso gummed on by a T-rex. This is why going from 0 to 100 in a car can either be no big deal (if done over 60 seconds) or kill you six ways from sunday (if done in a few milliseconds when you crash into a wall or something). We're also not getting into the power of a good muzzle break or the weight of the rifle itself, both of which considerably reduce the felt impulse by the shooter. You can look up people rapid-firing 50 cal rifles from the hip on youtube for an illustration of just how much you can tame the recoil of these guns.

CydeWeys 9y ago
A typical body armor plate has much more surface area in contact with the body than a rifle butt does though -- we're talking roughly an entire square foot. Said body armor plate, if it's to stop a .50 BMG round, also will be thick enough to weigh as much as an M82. This makes up for the fact that the acceleration happens over a reduced distance. Also keep in mind that a rifle needs to be comfortable enough for many repeated shots, while body armor only needs to be survivable. Note that you're at ~80 pounds from wearing just two plates to protect most of your chest and back; you'll collapse under the weight by the time you've armored your entire body.
- j9461701 9y ago
  For point of reference, shotgun slugs can be stopped by modern armor. It still cracks people's ribs through sheer impulse. A 50 BMG has 5 times the kinetic energy and 3 times the momentum of that. It's simply not enough to stop the bullet at these kinds of impulses, you need something to distribute the force over time as well as space. Cars use crumple zones for this purpose - the time it takes the front to deform is extra seconds over which the momentum change takes place, thereby decreasing the impulse. Even if we could develop some sort of powered armor that let you carry absurd amounts of armour I don't see how you could really solve this problem. The suit is still going to push back into the wearer's body with the same impulse, and crack bones when it does. Ultimately, the truth is human beings are just too squishy. #BringOnTheRobots
  - adrianN 9y ago
    If we allow for powered armor I can imagine armor with appropriate crumple zones. But I guess with powered armor you also get to carry ridiculously huge guns...
  - CydeWeys 9y ago
    Shotgun slugs are a lot less penetrative than .50 BMG though; they're proportionally heavier and travel slower. 12 gauge is .73 inches. You need significantly more thickness of armor just to stop the .50 BMG at all, by which point you've added so much more mass that you've gone much of the towards solving the higher impulse as well.
    - j9461701 9y ago
      >Shotgun slugs are a lot less penetrative than .50 BMG though; they're proportionally heavier and travel slower. What does the word "proportionally" mean in this sentence? Because factually, the slug is about half the weight of the BMG round. >You need significantly more thickness of armor just to stop the .50 BMG at all, by which point you've added so much more mass that you've gone much of the towards solving the higher impulse as well. You're looking at elephant-stomping-on-you levels of force, and you're trying to say it'll be mitigated somewhat because you have 5 pounds of steel on your chest rather than 2 pounds. You will be a person salsa even if you strapped a kitchen stove to yourself. You don't seem to be understanding the magnitude of the forces involved.
      - trhway 9y ago
        >You're looking at elephant-stomping-on-you levels of force, and you're trying to say it'll be mitigated somewhat because you have 5 pounds of steel on your chest rather than 2 pounds. You will be a person salsa even if you strapped a kitchen stove to yourself. the momentum conservation means that the 5 pound of steel would be moving with the speed of 18m/s after full momentum transfer from the 45g bullet hitting it at 900m/s. 5lb at 18m/s - is like somebody would hit you with a frying pan. While painful and dangerous if say into the head, it is still much less so than getting the bullet directly. Even better result would be with a 25 lb steel plate - just 3.6m/s - like somebody would shove an office/kitchen table at you. Elephant leg say weights 250kg - that would mean that if you have an elephant leg as an armor in front of you, after being hit by a 0.50 bullet with full momentum transfer the leg will be moving at the speed of 0.18m/s - you can just slowly step away from it. >You don't seem to be understanding the magnitude of the forces involved. please enlighten us and correct my calculations above.

CydeWeys 9y ago

A typical body armor plate has much more surface area in contact with the body than a rifle butt does though -- we're talking roughly an entire square foot. Said body armor plate, if it's to stop a .50 BMG round, also will be thick enough to weigh as much as an M82. This makes up for the fact that the acceleration happens over a reduced distance. Also keep in mind that a rifle needs to be comfortable enough for many repeated shots, while body armor only needs to be survivable. Note that you're at ~80 pounds from wearing just two plates to protect most of your chest and back; you'll collapse under the weight by the time you've armored your entire body.

j9461701 9y ago
For point of reference, shotgun slugs can be stopped by modern armor. It still cracks people's ribs through sheer impulse. A 50 BMG has 5 times the kinetic energy and 3 times the momentum of that. It's simply not enough to stop the bullet at these kinds of impulses, you need something to distribute the force over time as well as space. Cars use crumple zones for this purpose - the time it takes the front to deform is extra seconds over which the momentum change takes place, thereby decreasing the impulse. Even if we could develop some sort of powered armor that let you carry absurd amounts of armour I don't see how you could really solve this problem. The suit is still going to push back into the wearer's body with the same impulse, and crack bones when it does. Ultimately, the truth is human beings are just too squishy. #BringOnTheRobots
- adrianN 9y ago
  If we allow for powered armor I can imagine armor with appropriate crumple zones. But I guess with powered armor you also get to carry ridiculously huge guns...
- CydeWeys 9y ago
  Shotgun slugs are a lot less penetrative than .50 BMG though; they're proportionally heavier and travel slower. 12 gauge is .73 inches. You need significantly more thickness of armor just to stop the .50 BMG at all, by which point you've added so much more mass that you've gone much of the towards solving the higher impulse as well.
  - j9461701 9y ago
    >Shotgun slugs are a lot less penetrative than .50 BMG though; they're proportionally heavier and travel slower. What does the word "proportionally" mean in this sentence? Because factually, the slug is about half the weight of the BMG round. >You need significantly more thickness of armor just to stop the .50 BMG at all, by which point you've added so much more mass that you've gone much of the towards solving the higher impulse as well. You're looking at elephant-stomping-on-you levels of force, and you're trying to say it'll be mitigated somewhat because you have 5 pounds of steel on your chest rather than 2 pounds. You will be a person salsa even if you strapped a kitchen stove to yourself. You don't seem to be understanding the magnitude of the forces involved.
    - trhway 9y ago
      >You're looking at elephant-stomping-on-you levels of force, and you're trying to say it'll be mitigated somewhat because you have 5 pounds of steel on your chest rather than 2 pounds. You will be a person salsa even if you strapped a kitchen stove to yourself. the momentum conservation means that the 5 pound of steel would be moving with the speed of 18m/s after full momentum transfer from the 45g bullet hitting it at 900m/s. 5lb at 18m/s - is like somebody would hit you with a frying pan. While painful and dangerous if say into the head, it is still much less so than getting the bullet directly. Even better result would be with a 25 lb steel plate - just 3.6m/s - like somebody would shove an office/kitchen table at you. Elephant leg say weights 250kg - that would mean that if you have an elephant leg as an armor in front of you, after being hit by a 0.50 bullet with full momentum transfer the leg will be moving at the speed of 0.18m/s - you can just slowly step away from it. >You don't seem to be understanding the magnitude of the forces involved. please enlighten us and correct my calculations above.