Physics conudrum

[Zkribbler]

Putting spin on a bullet or shell keeps it going straight -- ballistics.

Putting spin on a baseball makes it curve-- curveball.

Can somebody please explain this to me?

[Dauphin]

Axis of rotation is different? A bullet's rotational axis is in the direction of flight, a baseball's is horizontal to it.

[KrazyHorse]

Baseballs have raised edges (the stitches) which drag air along with the spin of the ball.

Bullets and shells have very low raised spirals which channel the air past them when spinning.

[KrazyHorse]

Originally posted by Dauphin
At a guess, axis of rotation is different.

Nope. Curveballs curve left and right due to rotation about the direction of motion.

[KrazyHorse]

Now, that's why bullets and shells don't curve like a baseball. The benefit they get from curving comes from gyroscopic stability. The round will not tumble in the air as easily if it's spinning quickly.

[Dauphin]

Same with spinning tops if memory serves.

[KrazyHorse]

Originally posted by KrazyHorse


Nope. Curveballs curve left and right due to rotation about the direction of motion.

Actually, sorry. You were right. It's been a while since I threw a curveball.

Curveballs curve left and right due the component of rotation about an axis perpendicular to the ground. The Magnus force is nil when all rotation is about the direction of motion.

[Dauphin]

Cool, I was going to bow to your knowledge and assume I must have been wrong.

[KrazyHorse]

I had completely forgotten the action when you throw a curveball. In my mind I was remembering the baseball spinning like a bullet, but then that wasn't making any sense from a physics point of view.

The plus side is that now I remember how to throw one...

[KrazyHorse]

You have to break your wrist at the last second so that when you release the ball your palm is almost directly facing you. You should have twisted hard enough so that your thumb makes an audible "snap" into your palm...

[Dauphin]

Never pitched in baseball. I was thinking more of cricket, but that has as much to do with shine on the ball and weather conditions when in the air and pitch condition on bounce.

[Whaleboy]

What about the curve of a free kick in football? I would have thought that was effected by the change in relative air pressure on each side of the spinning ball?

OTOH would that only have an effect with an X axis spin as opposed to a Z-axis spin relative to the direction of motion?

[KrazyHorse]

Originally posted by Whaleboy
What about the curve of a free kick in football?

It has the exact same origin.

I would have thought that was effected by the change in relative air pressure on each side of the spinning ball?

That's an overly simplified explanation. I suggest you google "Magnus force" or "Magnus effect"

OTOH would that only have an effect with an X axis spin as opposed to a Z-axis spin relative to the direction of motion?

That's what I already said. The component of rotation in the direction of travel has no effect on the trajectory.

[Whaleboy]

That's an overly simplified explanation. I suggest you google "Magnus force" or "Magnus effect"

It's been a while, I recall it's analogous to the Bernoulli effect but I'll take a day or so to read up .

[Urban Ranger]

Why does a gyroscope resist changes to the direction of its axis?