What causes a curve ball to curve?

[Gonzolo]

If I try to explain it with friction, it doesn't make sense, because faster things have more friction. In curve balls though (ping-pong, trackball, baseball), the ball curves toward the slower side. So I can't seem to use friction as an explanation. What's going on?

 

[LURCH]

The spin of the ball is allowing air to move more quickly on the side spinning with the wind, and slower on the side that is spinning against the wind. The faster-moving air generates less pressure on the side that is spinning with the wind. This is called the Magnus effect, and is very much like the Bernoulli principal.

Interesting note: in an issue of Popular Science I read an article where some graduate students, using a fluid dynamics simulation program, proved that a curveball would curve the opposite way on Mars.

 

[Gonzolo]

Thanks, I knew there was a simple explanation.

So for Mars, I suppose it is because the density is less? Is my friction explanation above any good for Mars by any chance?

 

[LURCH]

Exactly! In the thinner atmosphere on Mars, there isn't sufficient air pressure to make a differential between the pressures on opposite sides of the ball, and friction becomes the dominant force.