November 03, 2004

*Ask Jen: Aerodynamic Lift Edition

Reader Stephanie writes, "My high school gym teacher says throwing a discus into the wind makes it go farther. Is he right?"

Well, as anyone who knows me can attest, physics is my real area of expertise.* Your teacher is right, because even though throwing the discus into the wind results in drag, it also gets an aerodynamic lift from the pressure differences on the top and bottom of the discus, which makes it fly for a longer period of time--and thus a greater distance as well.

* This is a blatant, big fat lie, but I did actually know the answer to this question. Yay, gym class! And if any of the real physicist types care to expound/clarify/correct in the comments, feel free.

Do you have a question for me? You can e-mail it. If I know the answer, I'll answer it. If I don't, I might make something up.

Posted by Jennifer at November 3, 2004 02:23 PM


You're entirely correct. The decrease in velocity is compensated for by the increase in lift, so that the longer flight time results in a longer distance despite the slightly lower velocity.

And yes, I am a Physicist, and I've taught both Physics and Astronomy at the college level, I don't just play one on TV...

Posted by: Jack at November 3, 2004 02:53 PM

Weight and surface area would surely affect the distance that the discus would travel, am I right?

A light discus would not travel as far against the wind, but once the wind caught it, it would travel with the wind in the opposite direction you wished.

Posted by: Oorgo at November 3, 2004 03:28 PM

Well, tossing a discus into Hurricane Francis might get you some lift, but it wont get you a whole lot of distance.

There's no equal compensation between lift and distance, Jack. A slight breeze might get you some lift (and, thus, distance) that more than overcomes the force of the oncoming wind. But, a strong wind could send the disc flying back over your head and smashing into the patio doors.

The same wind that's lifting it is also stopping it. To my feeble knowledge no one's ever devised a formula for downwind discus throwing. ;)

Posted by: Tuning Spork at November 4, 2004 12:31 AM

In fact, I'm willing to say categorically that the answer is "no". Air resistance is a hinderance. The force that gives the discus lift is giving it lift while (because) it's shortening it's distance. The lift is a drag.

Unless there's a perpetual motion machine out there, I don't see (right now) how a wind's lift can add distance to a discus.


Posted by: Tuning Spork at November 4, 2004 12:57 AM

Jack, you wrote:

"The decrease in velocity is compensated for by the increase in lift,"

"Compensated for by" should probably read "proportional to".
But, either way, the conclusion
"so that the longer flight time results in a longer distance despite the slightly lower velocity."

doesn't seem to follow. How do we get added distance out of a compensation? We seem to be getting something from nothing here.

Posted by: Tuning Spork at November 4, 2004 01:09 AM

I think you guys may be missing the most basic element of the issue. Lift is only generated if the air is passing over an airfoil - i.e. an object shaped like an airplane wing. The top surface MUST be shaped in such a way that the air takes longer to travel over the object than under it. The resulting lower pressure above the object creates lift.

A discus is symmetrical - same shape on top and bottom, therefore no airfoil, therefore the object is incapable of generating lift.

Posted by: Light & Dark at November 4, 2004 01:24 AM

Posted by: Jeff at November 4, 2004 01:46 AM

OK, Tuning Spork, I'll break out my textbooks on Bernouli's equations to make sure my immediate analysis was correct with respect to lift versus airspeed and drag as compared to the loss of ground speed.

I was referring to a normal wind and a normal frisbee (the kind I played with 30 years ago). There will be circumstances where there is so much wind it will blow the frisbee backwards, and circumstances where the frisbee is too light for what I said to be true.

I'll try to post the equations and the solution that (hopefully) backs up my statement on my weblog.

Posted by: Jack at November 4, 2004 03:41 AM

Paul beat me what I was gonna add here about the discus riding into the oncoming wind level. Unless the wind is blowing upwards or downwards the disc will ignore the wind liftwise., though it's still be slowed by a slight amount.

Frisbees and paper airplanes can get alot of distance because they are light and ride on the air -- kinda like an air hocky puck. A heavy discus will fly flat through calm air because air resistance will tend to keep it that way (path of least resistance).

Bottom line: The general statement that "a discus thrown into the wind will travel farther" is false, even though there are probably certain conditions where it can happen.

Posted by: Tuning Spork at November 4, 2004 11:33 AM

"A discus is symmetrical - same shape on top and bottom, therefore no airfoil, therefore the object is incapable of generating lift. "

I was going to make that point, but someone beat me to it.

The conditions where a discus could travel further into a headwind are

1) throwing the discus at the proper angle into the wind to force the air flowing over the top to travel at a greater rate of speed than what flows under the discus, thus generating lift, and

2) moderate headwind to prevent the wind drag from overwhelming the additional lift.

FWIW, I believe that the angle involved has to be fairly acute.

Posted by: physics geek at November 4, 2004 03:35 PM

To the kibbitzers here, I read "discus" to mean "frisbee" which is NOT symmetrical and DOES have lift associated with motion through the air. Also, keep in mind that lift is related to airspeed, not ground speed. So, what needs to be examined here is the change in lift from the increased airspeed because of the wind versus the decrease in ground speed due to higher drag from the increased airspeed.

As I said, I'll review the Bernoulli equations on lift and what I can find for drag on a frisbee and work out the math.

Posted by: Jack at November 5, 2004 02:50 AM

Or maybe just go outside and toss one a couple times? ;-)

Posted by: Jim at November 5, 2004 10:02 AM