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Liquid Armor the New Bulletproof Vest
kjh1 writes "Armor Holdings Inc. plans to start selling their 'liquid armor' next year. The new armor, originally envisioned to be spread on like peanut butter, is instead sprayed onto Kevlar in ultrathin coats. From the article: 'it's a mix of polyethylene glycol, a polymer found in laxatives and other consumer products, and nanobits of silica, or purified sand. Together they produce a "sheer-thickening liquid" that stiffens instantly into a shield when hit hard by an object. It reverts to its liquid state just as fast when the energy from the projectile dissipates.'"
But how much would it cost to coat your car in this stuff? And would it give extra protection?
Nope, not if it's your safety you're worried about, rather than the cars. You want the car to deform, so your decelleration slows down. Just like a helmet, you want it to break so you don't.
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If this is really what you bought it for (and actually do), Congratulations. You are one of the 0.5 % of SUV owners who actually should own an SUV. Unfortunately, 99.5 % of them are owened by soccer moms and men who need to overcompensate for something, and are just endangering us all on the roads, and burning very excessive amounts of gasoline.
Your logic is indeed false. First, it is arriving with a velocity and an associated momentum, not a force. Force is imparted on the bullet to change it's momentum and direction. Thus if it's really bouncing in the opposite direction, then twice the energy is required to stop it is imparted to the armor. See the sarcastic reply under yours for why this is unlikely the case. Most fo the time it would likely be deflecting off, requiring less energy than that required to stop the bullet. However, this still tells you nothing of the force. The peak force imparted to the bullet will be determined by the interaction of the bullet with the armor and contact time of the bullet with the armor. However, it's not even the force that you care about, it's the pressure imparted to the tissue. This will be determined by how the force is distributed over the armor. The force will be much greater if it's distributed over a 1 cm^2 area as opposed to a 1 m^2 area. To sum up, it's complicated and you can't draw conclusions of the performance of this armor by whether it "bounces off" or not.