Not so fast: Dropped bullet vs. Fired bullet

The fired bullet demonstrably hits the ground last, but it has little to do with the curvature of the earth. All objects moving through the air generate lift. You can make anything fly if you can just get it moving fast enough. This lift is enhanced by the rotational spin of the bullet. Hence a bullet fired from a gun will drop more slowly than one dropped from a hand.

Getting them to drop at the same speed requires conducting the experiment in a vacuum.

The Myth Busters disagree with you.

https://www.youtube.com/watch?v=abUBrQmI33Q

The test could be conducted in a vacuum or at the same barometric pressure with little or no wind currents.

The first paragraph of the answer in your link has this " (We assume a vacuum throughout this discussion.)"

Indeed.

It’s not necessarily wrong to fight the hypothetical. It’s generally wrong to fight the wrong hypothetical. :smiley:

I asked about dropping a bullet in a non-vacuum when this column appeared a few years ago. There was an interesting discussion in that thread (somewhat sidetracked by someone…challenged…by physics).

  1. The above quote is not true. The fact that the bullet is symmetrical on its axis of travel means that any lift generated upwards is identically counteracted by the lift generated downwards. Air foils generate lift because they are shaped differently on their top and bottom surfaces. A bullet has no way to know what direction is upwards.

  2. Cecil mentioned that “only when the distance is great, will the fired bullet land last. (paraphrased)” This is also not true. For ANY distance traveled, some curvature will exist. The only limitation to correctly identifying the fired bullet to land last is our ability to accurately measure time.

This is one of those things that everyone “knows” to be true but it really isn’t the whole story. Bournelli’s Principle may have some effect on slower than sound aircraft, but supersonic aircraft have very flat wings and gain lift purely from air deflection. It’s telling that even small fixed wing aircraft that have more curvature on the top of the wing than the bottom also have the wings angled to provide air deflection in straight ahead motion.

It’s been a long while since I have taken flying lessons, so I may be misremembering, but I recall my Cessna textbook from the late 80’s saying very much the same thing as I have tried to regurgitate.

Not so fast. Many aircraft have symmetrical wings. Aircraft flying upside down still manage to generate lift. Golf balls are dimpled for the precise purpose of generating lift, yet they are symmetrical. One of the reasons bullets spin is to generate lift. Even baseballs generate lift.

Sure, but golf ball lift is Magnus lift, which has to do with the rotation. The dimples just make the airflow more turbulent, which has the effect of giving the golf ball more lift than otherwise.

Wings generate lift through making a downward force on the airflow, which consequently provides an equal upward force on the wing, which is lift. In symmetrical wing shapes, this is accomplished by angling the wing upward a bit relative to the direction of motion, so that air hitting the bottom gets forced downward, and the plane gets forced upward.

Bullets don’t do either- they’re not angled relative to the direction of motion, and I suspect (IANA physicist) that any Magnus lift on a bullet if there is any, would be perpendicular in some direction to the bullet’s direction of motion, since the bullet is traveling along the axis of the spin, which is opposite of a golf ball, which usually has backspin, the axis of which is more or less perpendicular to the direction of motion.

And bullets spin entirely for stabilization, not for lift.

That does not mean that gyroscopic effects do not produce apparent lift.

You’re going to have to show a citation for that. Or maybe a recent fluid dynamics simulation.

Also, how do gyroscopic effects generate lift in a vacuum? (Reread Post 3, or the original article.)

Given the stipulations of Cecil’s original article, I’m afraid you’ll have to cite some other science besides fluid dynamics or aerodynamics for your support. And if you’re going to ignore the stipulations of Cecil’s article while arguing your point, I’m afraid you’re in the wrong thread and forum.

All wings gain lift entirely from air deflection, in accordance with Bernoulli’s Principle, which is really just a restatement of Newton’s second law for fluids. Low-speed aircraft have wings curved on the top because that’s the most efficient way to produce air deflection while keeping drag low.

And a spinning golf ball can generate lift, but a bullet cannot, because the golf ball’s spin is on a different axis than the bullet’s, and generates an up-down asymmetry, while the bullet’s does not.

When you angle a symmetrical wing upwards to create the deflection you speak of, the wing is no longer symmetrical along the horizon. The leading edge is raised and the trailing edge is lowered. Thus, the top and bottom halves of the wing (with respect to the horizon) are no longer symmetrical.

Any aircraft with symmetrical wings that generates lift, has those wings angled against the horizon.

The golf/baseball analogy does not work because the ball is spinning. If you threw a zero-spin ball and somehow managed to generate lift, you would have a point. However, that is impossible.

There are two working theories about lift generation: conservation of momentum (the air deflection you speak of) and the interactions of high/low pressure areas surrounding a moving wing. Both theories produce accurate predictions and neither one is “the only correct interpretation”.

Tell that to Phil Niekro.

In the thread I linked to above, Crafter_Man suggested that a bullet could generate some lift because its flight has a slight nose-up trajectory. In post #30 of that thread, ZenBeam links to this article giving the reason for the trajectory:

This article is describing a 45-degree initial trajectory. A bullet fired at 0 degrees will still travel in an arc, but is it enough to say the bullet will be nose-high? Also, the spinning of the projectile at a slight angle to its trajectory would cause a drift to the side, not up or down. But could the angle of the bullet itself actually generate any lift like a plane wing angled up?

What about, say, a Formula One race car? You know, the ones that have all kinds of spoilers designed to push the cars down harder onto the track ?
Or, heck, a curveball. The air-resistance force on a curveball is downwards (as well as backwards horizontally).

Sure it does. I pointed out that in my post above that golf balls and really anything with backspin are producing Magnus lift (or more accurately, lift from the Magnus effect).

A bullet would have to be spinning like normal while traveling sideways to get actual lift from the Magnus effect.

I suppose there is some generated by the bullet’s spin, but again, like I mentioned in my post, it would be perpendicular to the bullet’s direction of travel (and axis of rotation) due to the way Magnus effect lift works. The main effect would be to make the bullet less accurate.

what if you fired/dropped while standing on a treadmill?

(somebody had to say it)

Actually, I believe a spinning bullet wants to stay pitched upward (downward, if fired downward, but usually upward). I’m sure there’s some Magnus effect as well, but that’s largely perpendicular to the direction of motion, as has been pointed out.

Another source of increased drag is precession, which might be significant when comparing spinning and non-spinning bullets falling base-first.

That’s for relatively long rifle bullets. A tumbling pistol bullet may well exhibit less drag in general than one that’s falling flat end first.