+1 for sanity. There have got to be dozens of YouTube videos of people using a dremel or other motor to explode CDs with ZERO PPE or other safety equipment. IMO, a bullet is the wrong analogy to draw to start with, bullets aren't lethal because of the kinetic energy they impart, cars and C4 tend to be more lethal strictly from KE. IIRC, TNT is ~4 kJ / g, so using your calculation, if the rotor ran for 30 s and released ALL of that energy simultaneously, you'd get about 1.5 g of TNT. For the right incentive, I'd detonate 1.5 g of TNT in my bare hand and I'd stand next to a styrofoam box while 1.5 g of TNT detonated inside with little to no incentive. Just like a grenade, the danger doesn't come from the transfer of kinetic energy, the danger comes from the shrapnel's ability to penetrate clothing/skin. A razor blade with paltry amounts of energy can kill. A razor sharp piece of plastic would require phenomenal amounts of both energy and design to penetrate even modest metal plating.
On Sunday, March 4, 2012 8:39:10 PM UTC-6, Simon Field wrote:
On Sun, Mar 4, 2012 at 2:49 PM, Phil <philgoetz@gmail.com> wrote:Half (I think?) of the energy in a
bullet goes into the gun, and a gun is a lot less massive than a BSC.Well, no.In the limiting case of a rifle fired into the air, with it's butt resting on the ground,the rifle's motion is effectively zero, so all of the energy goes into the bullet.Energy is mass times velocity squared, and if velocity is zero, so is energy.In the case of a 2.25 kg pound rifle firing a 9.7 gram bullet, less than half apercent of the energy goes into the rifle, and more than 99.5% goes into thebullet.Anything that can knock a BSC weighing several hundred
pounds a few inches... well, do the math if you're curious, but I
doubt that a bullet could do that.Of course I'm curious, and the math is simple.The BSC mass is 272 kg.You say it jumped a few inches -- lets call it 10 cm.Let's say it came to rest in a single second, so the speed is 0.1 meters per second.The energy is then 2.72 joules.This is not surprisingly a fairly small amount. I can move a 600 pound box a fewinches without getting winded. Moving my own 200 pounds a few inches threetimes is not a lot of effort either.Calculating the energy in the centrifuge is also fairly easy, but to make it eveneasier, let's just ask how long it takes to spin up, and what size motor is usedto power the air compressor. A 200 watt (1/4 horsepower) motor, running for 30seconds is 6,000 joules of energy. The centrifuge system is probably 10% efficientat best at converting that electricity into rotary motion, as most of the energy goesout as wasted high speed air. Call it 600 joules. An AAA battery has 1,000 joulesof energy. A candy bar has a million.The muzzle energy of a .38 special is 420 joules.A .44 magnum gets you 1400 joules.Let's look at a 5 inch diameter rotor (I'm guessing at the diameter) spinning at 53k rpm,or 883 revolutions per second. The speed at the circumference (pi * 5 inches, or 40 cm)is 353 meters per second. That's in the same ballpark as a .22 rifle bullet. But that is thespeed of the very outside edge of the rotor. An inch in from the edge, you only get 200meters per second.
On Sunday, March 4, 2012 8:39:10 PM UTC-6, Simon Field wrote:
On Sun, Mar 4, 2012 at 2:49 PM, Phil <philgoetz@gmail.com> wrote:Half (I think?) of the energy in a
bullet goes into the gun, and a gun is a lot less massive than a BSC.Well, no.In the limiting case of a rifle fired into the air, with it's butt resting on the ground,the rifle's motion is effectively zero, so all of the energy goes into the bullet.Energy is mass times velocity squared, and if velocity is zero, so is energy.In the case of a 2.25 kg pound rifle firing a 9.7 gram bullet, less than half apercent of the energy goes into the rifle, and more than 99.5% goes into thebullet.Anything that can knock a BSC weighing several hundred
pounds a few inches... well, do the math if you're curious, but I
doubt that a bullet could do that.Of course I'm curious, and the math is simple.The BSC mass is 272 kg.You say it jumped a few inches -- lets call it 10 cm.Let's say it came to rest in a single second, so the speed is 0.1 meters per second.The energy is then 2.72 joules.This is not surprisingly a fairly small amount. I can move a 600 pound box a fewinches without getting winded. Moving my own 200 pounds a few inches threetimes is not a lot of effort either.Calculating the energy in the centrifuge is also fairly easy, but to make it eveneasier, let's just ask how long it takes to spin up, and what size motor is usedto power the air compressor. A 200 watt (1/4 horsepower) motor, running for 30seconds is 6,000 joules of energy. The centrifuge system is probably 10% efficientat best at converting that electricity into rotary motion, as most of the energy goesout as wasted high speed air. Call it 600 joules. An AAA battery has 1,000 joulesof energy. A candy bar has a million.The muzzle energy of a .38 special is 420 joules.A .44 magnum gets you 1400 joules.Let's look at a 5 inch diameter rotor (I'm guessing at the diameter) spinning at 53k rpm,or 883 revolutions per second. The speed at the circumference (pi * 5 inches, or 40 cm)is 353 meters per second. That's in the same ballpark as a .22 rifle bullet. But that is thespeed of the very outside edge of the rotor. An inch in from the edge, you only get 200meters per second.
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