Here is a cool video of a bullet hitting a metal plate in super slow-mo. I’m wondering if anyone can explain why the speed of the shock wave is so much faster than the speed of the bullet? I’m sure it’s got something to do with Newton, but I can’t figure out why something traveling at a certain speed would cause something else to travel at a faster speed. Are there other common examples of this happening that would make it seem more obvious why?
A shock wave isn’t like a single thing moving by itself. Sound is a shock wave where compression waves travel through the air - the air isn’t moving, but each particle bumps up against its neighbor, which bumps up against its neighbor. (And, therefore it makes sense that density affects the speed). Each individual particle is moving only a little bit and not very quickly (think of fans in a sports stadium doing a wave). Shock waves have a speed that is based on the density of the material, so you’d expect it to go pretty fast through metal.
Maybe the best example is clapping your hands. You hands are moving only a few miles per hour, but the sound wave that you create travels at the same speed as any sound - several hundred miles per hour.
In the bullet example though, there seems to bet physical debris riding the shock wave- is it possible for a sound wave to carry dust?
Just a guess, but is it because the energy imparted on the subset of particles moving perpendicular to the bullet is large enough to move those particles faster than the bullet itself (but the total energy is conserved because this is a subset of the bullet mass)?
Another analogue - hold a melon pip between your fingertips and squeeze - the pip will squirt out at a higher speed than your fingers were travelling as they closed together on it.
That’s what’s happening with the material in the video, I think - it’s being squirted out sideways by the pressure of the bullet - it’s an example of mechanical advantage.
Yes. For some hard data:
- speed of sound in air is about 350 m/s
- speed of bullets range from subsonic to some 400 - 450 m/s for handguns and up to some 950 m/s for rifle
- speed of sound in iron is 5100 m/s - so shockwave will propagate through metal target at least five times faster than bullet speed.
Judging from the video though, there doesn’t appear to be a noticeable shockwave going through the metal. If there is, it is obscured by the explosion of debris & air heading radially outward from the point of impact.
It’s also why the physics of hypervelocity impacts like meteoroid strikes is different. If the impact velocity is higher than the speed of sound through either the projectile or the target, energy doesn’t have time to be carried away and the projectile and it’s immediate target basicly vaporize and explode.
I don’t think you’re really going to be able to see the shock wave in the target. Partially because it’s going to be going through the material as p-waves perpendicular to the surface, and the s-waves aren’t going to be strong enough to distort steel in the lateral direction.
I’ve been involved in dozens of tests where we were shooting massive projectiles into concrete walls, and have seen hundreds of steel plates that were impacted by projectiles. The high speed cameras were much higher quality than the one in the linked video (or at least the final product was. I think we were shooting around 5,000 frames per second, but it’s been awhile) Admittedly, most of the tests were only around Mach .8 or so, but even the bullet example is going to be going slower than the speed of sound in the projectile or the target.
One thing that’s quite interesting about concrete is the way it spalls when subjected to a high speed impact. What happens is that the wave travels through the target at the speed of sound in that material. Since the bulk sound speed is generally a few times faster than the projectile speed, the wave reaches the back of the target when the projectile has only traveled between ten and fifteen percent of the way through the target. When the wave hits the back of the target, it is reflected. However, the impulse is going to be transferred into the target as long as the projectile is still moving through it. So, the initial wave intersects with the reflective wave, basically doubling it’s amplitude. To make a long story short, the back of the concrete is spalling out before the projectile is even a quarter of the way into the target.
I’ve also worked on a light gas gun. The physics are similar at impact, but I’ve never been on one that had portal for a camera, so I haven’t literally seen any of the 5 km/s shots
A very simple example would be pressure: fill a plastic bag with water and poke a small hole in it. Even pressing slowly on the bag will cause water to shoot quickly from the tiny hole.
That could be what’s happening with the material in the video: pieces of material much smaller than the bullet are getting compressed and then ejected at high speed.