1. 2011
Jun
08

## Spinning bullets

While catching up on some old Mythbusters episodes, I ran across an interesting myth about spinning bullets. Apparently when you shoot a bullet into the surface of a frozen lake, it bounces backward, bounces over the ice a little way, and keeps spinning even after it comes to rest.

This is a very curious result. At first, it kind of seems to make sense. A bullet comes out of the gun spinning at 80000 revolutions per minute, according to the show, which is pretty fast. So it makes sense that it has a sizable amount of angular momentum, which in turn means it’ll take quite a bit of torque to stop it. Colliding with the ice conceivably might not be able to exert enough torque to do that, so the bullet would keep spinning.

But if that were the case, the bullet should maintain its orientation — it’d keep pointing in the same direction that it came out of the gun, because angular momentum is a vector quantity and it won’t change in either magnitude or direction without an external torque. That clearly wasn’t the case; the bullets the Mythbusters shot skittered across the ice, spinning in …

2. 2010
Jan
07

## Bouncing Bullets

Whenever Mythbusters meet bullets — no, not literally, though this week’s episode of Mythbusters does have Adam and Jamie trying to shoot cardboard cutouts of themselves — you know something wacky and interesting is about to ensue. The myth in question is that, with an unwisely aimed shot, it’s possible for a bullet to bounce off three steel beams and come back to hit the shooter.

Seems straightforward enough, right? If the beams, or walls as the case may be, are lined up at right angles to each other, why shouldn’t a bullet just bounce off all three and come right back to where it started?

As Adam and Jamie (re)discovered during the show, bullets don’t bounce, at least not when they’re moving as fast as, well, a speeding bullet. They shatter on contact with any hard enough surface, like steel, and the pieces spray out in what could be a completely different direction from what you’d naively predict.

From a physics perspective, this highlights the difference between elastic and inelastic collisions. Elastic collisions are based on the idea of a ball bouncing off a wall; it goes in at some speed and bounces right …

3. 2009
Dec
18

## Unarmed and unharmed

This is one of those really cool things that I’ve often wondered about: can you really shoot a gun out of an outlaw’s hand? Last week on Mythbusters, Adam and Jamie decided to test it out. Sure, it’s not the kind of thing you’d think would be easy (or safe) — unless you have access to that classic Mythbusters creativity. Their first idea involved a Velcro-like gripping arm to hold the gun, and although it may not be clear just how exactly that compares to a real hand, they obtained some interesting results from comparing the different gripping positions.

Anyone who’s ever tried to pry an object out of somebody’s hand knows that the easiest way to do it is to twist it to apply stress on the thumb, the weakest point of the grip — not just to hit it as hard as possible. And whenever an object is twisting or rotating, the operational physical principle is torque, the rotational analogue of force. Torque can be calculated from the formula

$$\vec{\tau} = \vec{r}\times\vec{F}$$

but in most simple cases, we can identify an axis of rotation and then calculate the torque around …

4. 2009
Oct
10

## Bullet Fired vs. Bullet Dropped

With their season premiere this week, the Mythbusters are testing a classic physics story, so of course I had to comment on it. The myth in question is that if you fire a bullet from a gun held horizontally, it will hit the ground at the exact same time as a bullet dropped without any horizontal motion at all.

Of course, in the mind of any physicist, this is no myth at all — the laws of physics that tell us this should happen are so well established that they’re almost beyond question. Specifically, it’s the linear independence of orthogonal vectors, which means that components of motion that are perpendicular to each other, like gravity (vertical) and constant velocity (horizontal), don’t get in each other’s way. You can split the motion of the bullet into two perpendicular components and analyze each one separately. This is, in fact, one of the first things students learn in an introductory physics class: analyzing the motion of a fallen or thrown object. The equations $$x = v_{0x}t$$ and $$y = -\frac{1}{2}gt^2$$ work for both the fallen bullet and the dropped bullet, just with $$v_{0x} = 0$$ in …

5. 2009
Jun
11

## Curving bullets

This week the Mythbusters tackled the question of whether you can make a bullet follow a curved flight path, as in the movie Wanted. The characters in the movie are able to do this using some fancy flick of the wrist as they fire the gun, but is it really possible? Apparently a lot of people were wondering.

The short, simple answer is no. It’s an obvious application of Newton’s first law of motion: objects moving in a straight line will continue moving in a straight line at constant speed, unless subject to an external force. But there are only a couple of external forces that can act on a bullet: air resistance and gravity. Gravity certainly isn’t going to make the bullet curve sideways as we see in the movie, and all air resistance will do is slow it down, not change its direction.

Then again, Kari, Grant, and Tory hit on an important point: bullets are highly symmetric and are typically ejected from the gun barrel with high spin. All this is optimized for motion in a straight line toward whatever you’re aiming the gun at. What happens if you use asymmetric, oddly shaped …