# The air drag and magnus pass spin axis?

1. Oct 25, 2008

### zyh

The air drag and magnus pass through spin axis?

A ball is flying in the air, and it has spin on y axis, show below.

F1 is the air drag force.
F2 is the magnus force.

My question is: the air drag and the magnus force is pass through the spin axis? Or, the ball will change the torque direction. Thanks.

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Last edited: Oct 25, 2008
2. Oct 25, 2008

### Phrak

Are you asking if these forces are passing through the spin axis?

There will be an unevenly distribution of forces acting on the surface of the ball in the xz-plane. Together, these can be decomposed into a force couple with torque about the y-axis, in addition to an unbalanced force. The unbalanced force is both lift and drag.

In general, the unbalanced force would not act through the center of the ball.

It's kind of a no brainer that the viscous drag forces will show the rotation. This would leave the lift and induced drag to consider.

Last edited: Oct 25, 2008
3. Oct 25, 2008

### zyh

Yes, Thanks Phrak for your hint, I have changed the original post.
BTW, I'm not an English native speaker, so, I'd try to improve my written English ability.^_^

4. Oct 25, 2008

### Phrak

Read it again. I've been editing. We've each been writing to moving targets.

Last edited: Oct 25, 2008
5. Oct 26, 2008

### zyh

Did you mean that the distribution of forces can decomposed to two parts?

One force is couple with torque about the y axis, which will slow down the rotation?
The other force is " lift and drag", which will slow down the motion and get a curve track.

Another question is : After a period of time, the spin axis will still be parallel to the y axis?
Thanks.

6. Oct 26, 2008

### Phrak

7. Oct 26, 2008

### zyh

thanks
I read the web page your suggest: Equivalent force systems.
I do agree with you that there are three force:
gravity, lift, and drag.
and there is a couple

You said
I think it is right, because there is no reason that every
But if the spin axis is perpendicular to the ground, and is perpendicular to the Velocity of the ball( see my picture, the Y axis is the spin axis, and the Z-X plane is the ground plane). In this case, the spin axis will vary during several time?

8. Oct 26, 2008

### Phrak

We're talking baseball, right? I don't think, in a pitch, the axis will change more than a couple degrees in a normal throw. If the initial spin is slow enough, it may be significant. I dunno.

9. Oct 26, 2008

### zyh

I mentioned table tennis which is the same as baseball I think. I'm trying to detect a spin ratio of the flying table tennis ball through it's trajectory. If the spin axis doesn't change it's direct, it may work, otherwise, it will be more complicated.

10. Oct 27, 2008

### Phrak

Well, that gets interesting. The dynamic forces on the ball are large compared to its moment of inertial. But this takes the problem outside of my scope.

A wiffle ball ping pong ball could get real intesting. Poke some holes in one and see what happens.

...It occurs to me that the aeronautical engineers around here could be more helpful. In this folder:

https://www.physicsforums.com/forumdisplay.php?f=187

Last edited: Oct 27, 2008
11. Oct 27, 2008

### zyh

Thanks for your suggestion.
I'd prepare a new post there.

12. Oct 27, 2008