It appears we have yet another “Matrix-like” action fantasy violence-fest in the just-released movie Wanted starring James McAvoy, Angelina Jolie and Morgan Freeman. The curving-bullet scene we see in the trailer is fast becoming a topic of conversation in certain moviegoing circles (i.e. high-school kids). It appears that McAvoy applies a little “English” to the bullet with that fancy flip of the wrist. Now, clearly the scene is just silly fantasy –curving a bullet a couple of feet around a butchered pig does not conform to the physics of the known universe. However, the interesting question is whether or not it might be possible to manipulate the gun to impart any kind of curve at all.
Ballistics is a rather complicated topic. When a bullet is fired out of the barrel of a gun it can have a velocity of up to 1000 m/s and a rotation rate up to hundreds of thousands of rpm. It is immediately subjected to the downward pull of gravity, and a large air-drag force slowing the bullet down. While the rotation stabilizes the bullet by the same principle that a spinning gyroscope is hard to knock over (conservation of momentum), this same rotation can result in a small upward or downward force along with a sideways drift as a result of the Magnus force. The Magnus force is the same force that makes it possible to throw a wicked curveball—the result of air traveling at different speeds around each side of a rapidly spinning ball, thus creating a different air pressure on each side. Of all of these forces, gravity has the largest effect on the bullet’s trajectory. Over a distance of a five or ten meters like that shown in Wanted‘s slaughterhouse scene, the combination of these forces would result in a deflection of a few millimeters— obviously not sufficient to clear Angelina (and the pig) and hit the target.
Acknowledging all of the above, the question that we still need to address is: Could we get any additional help from the wrist flip? Another millimeter? Anything? The answer is (not surprisingly): no, we could not. The reason is very simple and has to do with Newton’s first law of motion. The implication in the scene is that the curving trajectory of the gun is somehow imparted to the bullet before it’s released and the bullet continues to curve after it leaves the barrel of the gun. That’s not the way bullet-firing works. It’s a common first-year physics misconception, for example, that a ball rolling around a curved track will continue in a curved path after it leaves the track. It won’t. The reason an object follows a curved trajectory is because there is a force pulling it into that trajectory—for instance, the walls of the track pushing on the ball. As soon as the forces stop acting, the object is going to continue in a straight line at a constant speed, according to Newton’s first law. Try this at home: Attach a string to a ball and swing it in a horizontal circle just above the ground, and then release it. Notice the path it takes after you release it. It’s a straight line. Same thing with the bullet after it leaves the gun.
Adam Weiner is the author of Don’t Try This at Home! The Physics of Hollywood Movies.