Welcome to the Straight Dope Message Boards, Ones, glad to have you with us.
When you start a thread about one of Cecil’s columns, it is helpful if you provide a link to the column. Yes, it’s on the front page today, but it will soon disappear into the Archives, and then people won’t know what you’re talking about… No biggie, but it helps keep everyone mostly on the same page.
The baseball and the beachball both break out as Cecil outlines. Some texts used to say baseballs curved in the opposite direction because that’s what Bernoulli’s Principle predicted. There are still people that believe Bernoulli’s Principle provides lift to airplanes and causes balls to curve.
Soccer balls are air-filled, and break in accordance with the principles stated in the article. Kick the ball on the right side to make it spin right to left.
Golf balls, btw, probably are the easiest example of the various spins that one can impart and their result, since we are all quite familiar with watching hooks and slices (well, at least I am, having hit numerous of them in my days).
The use of Bernoulli’s Principle to explain airplane lift assumes that air flowing over the top and bottom of the wing reaches the trailing edge at the same time. Wind tunnel tests have shown this not to be the case.
The exclusion of Bernoulli’s Principle to explain airplane lift assumes that the air flowing over the top and bottom of the wing are magically synchronized so that the linear rate of airflow over the two surfaces are the same, which requires not only spooky action at a distance but violation of several conservation laws.
The standard “Mr. Wizard” exposition of the matter is oversimplified, God knows (i.e., airplanes demonstrably can fly upside-down), but the fact remains that airfoil designers find that the predictions of Bernoulli’s Principle are accurate in practice.
John, I’ve never seen lift explained by the “equal linear rate” model. If you provide a cite, we can both make fun of it. Also, from what I’ve read, airfoil designers don’t use Bernoulli to help with lift calculations. They use a “circulation” model.
I was happy that Cecil in this column used the intuitive air deflection explanation instead of Bernoulli. I have a feeling that ten years ago, the same subject would use Bernoulli. The trouble with it is that to calculate the pressure difference, you’d have to know the speed over the top, and without the equal transit times assumption, how do you arrive at that? You’re probably not going to design any airliner wings with the air deflection idea, but it’s great at explaining lift qualitatively. Beyond that, you have to get so deep that you would need expertise in computational fluid dynamics.
Baseballs, yes. Soccer balls, yes. Impart counter-clockwise (when viewed from above) english and they bend/curve/break right to left.
What I’m saying is, if I kick a beach ball in a similar fashion–counter-clockwise english (wvfa)-- it does NOT break right to left but left to right. (Might it have something to do with the fact that I’m kicking the beach ball instead of throwing it?)
OnesNZeros: I am only partially apologetic for this hijack. 1) Our goal is to irradicate ignorance. 2) This topic might help explain your question.
I was unable to find the Cecil column that stated that golf balls curve because of Bernoulli’s Principle. I was unable to find the threads in response to that column.
In actuality, the airfoil designers use LIFT formulas, not BERNOULLI’S formula.
Lift formulas (L = Cl x r x V[squared]/2 x A): are based on experimental evidence and include ALL sources of lift.
Bernoulli’s formula (P + ½ v2 + gh = constant ) only accounts for ~5% of lift, even on wings with especially developed “humps”. This is why airplanes can fly upside down: it is easy to overcome 5% negative lift, just increase your angle of attack. Now if you are talking STOL (Short Takeoff and Landing), 5% Bernoulli will decrease your stall speed by…5%, but if you are talking high speed flight, then the turbulence from the hump is actually detrimental (use your flaps at low speed) and anyway, Bernoulli is redundant at high speed: soooo much energy available for deflecting air downward.
Bernoulli’s Principle (Note that this website mistakenly spouts the dogma that Bernoulli is the source of lift): The Pressure over a surface is directly proportional to the speed of the fluid over the surface.
It is clear from this definition and this formula, that for Bernoulli to be effective, the speed of the air over the top of a wing must be considerably greater than the speed over the bottom of the wing. Experimental data show that it is only slightly faster – Only enough to account for about 5% of the total lift.
Look at Slug’s diagram: If you spin the ball so that the speed of the surface exactly matched the throwing speed of, say, 90 mph, then the upper surface meets the oncoming air at 0 mph, while the lower surface meets the air at 180 mph!!! Bernoulli predicts that the great “speed of the fluid” (air) will cause a great “decrease in the (air)pressure”: this ball should drop like a rock. And yet the ball maintains its altitude and does not drop. Newton rules!!
Bernoulli is only worth a footnote. You either see this or not. And it is so much easier to see this if you have not been indoctrinated with Bernoulli.
According to Slug’s diagram, if you kick a ball with a clockwise spin, the ball curves right to left, a counterclockwise spin will give a left to right curve. In my experience, ALL balls follow this curve.
Can you make a tape of a counterclockwise spun ball curving right to left? I would like to see that.
Well, the air in a beach ball 20 inches in diameter would weigh 3 ounces or so. I suspect it would be tough to find a ball whose uninflated weight was that low, but a really flimsy one might not be a whole lot heavier than that.
Yet it’s hard to see how this suspends the magnus effect.
I’m going to have to because it seems, much to my surprise, that my experience on this is isolated.
And an aside to whomever aololgized for hijacking the thread: not to worry. I’m just as curious–if not more so–about the exact reason airfoils generate lift as I am why beachballs hook one way off my foot but another off everyone else’s.
I can’t believe nobody is seeing the genius in my post, as I’m getting more and more convinced that’s the crux of it.
I said the air outweighs the surface of the ball (well, actually I was careful to talk about mass, not weight, but whatever), and I shouldn’t have - there’s nothing magic about passing the point where 50% of the mass is air and 50% is surface. But the proportion of air mass to surface mass in a beach ball is much much higher than in a soccer ball.
Have you ever twisted a glass containing a drink around? The drink tends to not rotate as the glass does. Same thing with trying to spin an egg- the weight of the contents doesn’t spin, thus the egg flops over.
I think any spin you try to put on a beach ball soon dissipates - and the ball behaves like a knuckleball - even an imperceptible left-to-right airflow would carry the ball.
Further, I’d like to know if our thrower is side-arming, to get the real dramatic slider action. The initial outward path of the ball, which the air inside the ball would keep as momentum, would help the left-to-right.
I accept that a beach ball won’t hold its spin as well as a heavier ball. But you can watch the thing and see if it’s spinning or “knuckling”.
As I understand it, OnesNZeroes is saying that the ball is spinning, but breaking opposite to the usual sense. If he’ll add that the break happens when the spin stops, then your idea may explain things.
OnesNZeros: I watched a great show on the Science Channel last night. Projectiles: Boomerangs to Balls It will probably repeat today. They showed wind and water tunnel streams around assorted balls and talked about curves. Baseballs and tennis balls and golf balls (and beach balls) follow slugs diagram.
I’m still inclined to believe that you saw a diagram of balls curving right to left when thrown counterclockwise as an example of Bernoulli’s principle. I have seen this myself. I find it amazing that this happens.
It’s not the mass or the weight or whether the interior is fluid: It is the density. ALL BALLS curve as the beach ball, the beach ball curve is simply an exagerated version of the curve. A beach ball filled with Aerogel would curve exactly the same.
JWK: As they were explaining the Boomerang’s lift, they showed the classic airfoil and said: ~“the Boomerang gets its lift the same way airplanes and birds get their lift”.
“Great”, I thought, “another generation of JWK’s indoctrinated into Bernoulli.” Imagine my surprise when they showed the air pressure increasing under the wing by the angle of attack and decreasing over the wing by the deflection of the air away from the surface. Not even a Bernoulli footnote.
BTW - The Bernoulli component of lift on a wing is actually closer to 2.5%.