The air drag and magnus pass spin axis?

[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.

 

[Phrak]

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

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.

 

[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.^_^

 

[Phrak]

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

 

[zyh]

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.

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.

 

[Phrak]

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.

That's about it, yes. The lift can also act against the force of gravity if you include it.

http://emweb.unl.edu/negahban/em223/note8/note8.htm

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

If all the initial forces: lift, drag, mg, and the force couple act in the same plane, the spin axis won't process. This would be the case with the spin axis horizontal to the ground.

 

[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

If all the initial forces: lift, drag, mg, and the force couple act in the same plane, the spin axis won't process. This would be the case with the spin axis horizontal to the ground.

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?

 

[Phrak]

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?

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.

 

[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.

 

[Phrak]

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.

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

 

[zyh]

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

 

[zyh]

HI, Phrak, I add a new picture(one picture is more than thousand words,) and post there:
https://www.physicsforums.com/showthread.php?p=1931663#post1931663

Thanks