On Mars, Curveballs become Screwballs

Space.com ^ | 21 February 2003 | Robert Roy Britt

[Black Powder]

If a baseball team traveled to Mars for an interplanetary away game, shortstops and second basemen would become instant sluggers, benefiting from the reduced gravity and thinner air.

And according to a new study, pitchers might find their curveballs behaving like screwballs. The reverse behavior would owe to Mars' practically nonexistent atmosphere and the complex "fluid dynamics" that make a spinning ball curve.

Finally, to the delight of the hitters, tricky pitches on Mars would not be nearly as lively as here on Earth.

Worlds apart

In rarefied air, with molecules far apart, a whirling object can actually drift in the opposition direction compared to what happens under normal terrestrial circumstances, according to a new study that, inherently, had nothing to do with Mars or space. As it turns out, the reverse physics might also affect spinning spacecraft in the upper reaches of Earth's atmosphere.

The research is detailed in the March issue of the journal Physics of Fluids.

A standard curveball, thrown by a right-handed pitcher, is generated by a flick of the wrist that imparts spin on the ball. Viewed from above, the curveball spins counterclockwise and curves in the same direction as the spin, to the left. A screwball is spun clockwise and curves right.

High-paid athletes might not think much about why a curveball curves, but scientists know the answer involves speed, drag, pressure and Bernoulli's law.

The spin

Here's how a curveball works, again looking at it all from above:

A spinning object creates a whirlpool of air around it. On the left side of a curveball, this whirlpool is moving in the same direction as wind that's zipping past the ball, generating increased air speed. On the right side, the whirlpool opposes the oncoming wind, slowing it down.

As any airplane wing designer knows, faster-moving air means less pressure (wings are designed to make air move more quickly over the top, thus providing lift). With our curveball, the left side experiences less pressure than the right side, pulling the pitch to the left.

The phenomenon is called Magnus force.

Going the other way

In thin air, however, the whole whirlpool process breaks down if the distance a molecule must travel to hit another molecule is greater than the diameter of the spinning object. In this case, another process governs the ball's movement.

Now, sans a whirlpool, our intended curve ball interacts directly with incoming air.

"The side of the ball facing the incoming gas molecules will deflect these in the direction given by the rotation," researcher Hanno Essen, of Stockholm University, explained in an e-mail interview. "The ball will therefore (according to the law of action and reaction) tend in the opposite direction." Meaning the molecules go left, the ball goes right.

While Essen and his colleagues did not project their findings to baseball games on Mars (perhaps because baseball is not that popular in Sweden), the folks who run the online version of the journal Nature did. Actually, being based in London and favoring a different sport, they projected that even a soccer ball on Mars would arc in the opposite direction a kicker would expect.

Easier to hit

Essen notes that the force on an object will be quite small if the gas is thin: "So, even if the baseball might curve in the opposite direction, the curving would be very small compared to the ordinary Magnus-force curving on Earth."

The reverse-Magnus-effect can even occur on Earth, Essen said, with very tiny "macro-molecular" objects. And while the study examined objects moving at small velocities, Essen figures there is reason to believe that it will govern things at higher speeds and thus be relevant for re-entry of spacecraft.

The study was led by Karl Borg at the Royal Institute of Technology in Stockholm.

[arly]

woww I'll have to remember this the next time I travel to Mars!

[hole_n_one]

You can establish any set of atmospheric conditions and see what effect it will have on a thrown curveball at this NASA site.

[hole_n_one]

The NASA site allows you to set up the atmosheric conditions for Mars via the location tab.