## Tuesday, July 11, 2017

### That's some weird bullet behavior!

I received the link to this video clip via an e-mail list to which I belong. Watch it in full-screen mode for best results.

Why would a bullet behave like that when fired into a sheet of ice?  I can't figure out the physics of it.  Can anyone help?

Peter

ASM826 said...

Ever play with a top? It's the same physics.

The bullet is rotating as it leaves the barrel. if it impacts the ice at a steep enough angle but doesn't penetrate, it loses it's forward momentum, but retains the energy of it's rotation. The surface of the ice has very little friction in relation to the tip of the bullet so it continues to spin a long time.

Spinning a top

camperbot said...

Looks like the impact with the ice flattened the nose slightly. It then popped out and landed on the nose. Because it was still spinning fast enough around its long axis, it then behaved like a small spinning top.

There's an informative page here on spin rate and stability:

http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/

You could plug in an average 9mm's dimensions and muzzle velocity into the formula on this page and get a couple of thousand rpm for the bullet, more than enough to make it behave like a top.

bmq215 said...

Yep, the stability of that spin is the reason why rifling is so important.

That it lands point down is probably due to the fact that A) it was pointed in that direction to begin with and obviously ricocheted back in the direction of travel, B) any slight off-axis motion component was corrected for by the spin, and C) point-first is, by design, the most aerodynamic way for it to fall.

Dropping a spinning object often results in this outcome. You can try this yourself by dropping a quarter on your desk while imparting spin with the thumb of one hand and the index finger of the other. You'll see it land off angle, stabilize, and then go into a clean spin (sometimes after a bounce or two).

Chris Mallory said...

Mythbusters Season 9 Episode 7 had this experiment.

Giving the bullet room to move backwards, they were able to get the bullet to spin 3 times in a row. But their bullet was on it's side, not it's point.

Anonymous said...

And I wouldn't be surprised, if there would be between the bullet and the ice a microscopic small patch of liquid water, or maybe even steam!

Knolli

Antibubba said...

A 45 ACP would've shattered the entire pond!

;)

Anonymous said...

And extremely low friction for the same reason as ice skates - the contact patch is water (caused by heat rather than pressure).

willy kanos said...

Doing a little math here: As ASM826 said, the ice stopped the forward momentum but not the spin imparted by the rifling. The math: If the bullet left the muzzle at 1,000 fps and the barrel rifling was one turn in ten inches, which is common but not universal for 9mm parabellum, the bullet would be spinning at 100 revs per second or 6,000 rpm. That's far faster than the child's top linked to. As you saw in the top video the man threw the top upside-down but it found its stability upright with point down. The rotation causes the stability just as it does with a gyroscope.

Uncle Lar said...

Liquid water is incompressible but ice is less dense with a crystal structure. That's why ice floats.
I would have assumed that the bullet would glance off the surface and scream off into the distance, but at the angle of the shot and perhaps a bit of an ice ridge the bullet point dug in and burrowed that hole. Forward momentum dissipated until the bullet stopped and was actually pushed back out by the expansion of the ice and possibly a bit of steam as the slug would have still been very hot at that point. There were minimal forces there to slow rotation, in fact any ice in contact would have flashed to liquid or gas and served as a lubricant.
The part I find fascinating is that as it popped out of the hole it assumed a point down orientation and gyroscopically maintained that for as long as rotation lasted.

Anonymous said...

It's not a trick either. After seeing this on the internet a couple of years back we tried it on the pond and the creek in the winter and it does indeed happen on occasion. We tried a couple different calibers and we only got the 9mm to do it for some reason. Rifle rounds were too violent. I thought .45 might but we ran out of FMJ before we got it to work.

Anonymous said...

Well, I think this should be applied for a 50 million grant to find out if ANY of these answers are correct or maybe something entirely different is effecting the bullet, like global warming or one of the clittons.
Heltau

jon spencer said...

Was there when that happened while ice fishing. A .22 pistol was fired and the bullet was spinning on the ice surface while on its point. The bullet spun for quite awhile too.

Roy said...

Had it happen with .45 hardballl on a creek. Bullet bounced strait up, landed on the ice four seconds later, spun for a good two minutes on its nose. A spinning object will get up and spin on its long axis. Try it on a countertop with a hard boiled egg. ( A raw egg has liquid turbulence and will not spin. )