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PhilG
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I've noticed that a football (American) wobbles a little bit even when you throw it with a near perfect spiral. Why is this?
Aerodynamic drag forces are not perfectly symetrical so they create torque on the ball. This means that dL/dt is non-zero so the angular momentum is not constant. So it has to precess. Even bullets precess slightly (bullet yaw). It can be minimized but not eliminated.PhilG said:I've noticed that a football (American) wobbles a little bit even when you throw it with a near perfect spiral. Why is this?
Seems like it would be...PhilG said:If the ball is tumbling and spinning at the same time, isn't its angular momentum changing?
It doesn't rotate end over end if you put spin on it. It does wobble though.PeteSF said:Toss a pen in the air so it rotates end over end.
Now repeat, but put a spin on the pen as you toss it.
What do you actually mean by "rotation on two axes"? The result has to be equivalent to rotation about one axis (not necesarily a principal one). Would you say that axis of rotatoin is moving or fixed?There you have it. Rotation on two axes. No torque applied after the initial impetus.
It surely does. I'm doing it right now. Try it again.HackaB said:It doesn't rotate end over end if you put spin on it. It does wobble though.
I mean two components of rotation, with two different periods.What do you actually mean by "rotation on two axes"?
Why?The result has to be equivalent to rotation about one axis (not necesarily a principal one).
One axis (the end-over-end rotation) is moving in a parbolic arc, but always points in the same direction.Would you say that axis of rotation is moving or fixed?
U are right, I was putting too much spin and not enough end-over-end impulse on it. However, the faster you spin it, the more it deviates from a pure end over end rotation. It wobbles in a sideways direction too. I don't think you can avoid it, but I could be wrong.PeteSF said:It surely does. I'm doing it right now. Try it again.
HackaB said:What do you actually mean by "rotation on two axes"? The result has to be equivalent to rotation about one axis (not necesarily a principal one).
Well, if an axis is "moving" and the ball is not, then that axis (of rotation) must be, itself, rotating. i.e. two axes of rotation.Would you say that axis of rotatoin is moving or fixed?
I see what you mean now. Can you explain how the angular momentum stays constant while the angular velocity vector moves around in a complicated way?DaveC426913 said:No. Rotation about two axes is not equivalent to rotation about one axis - even if you choose it carefully. The football is an excellent example:
I float around in the shuttle bay.
I take the ball and put a strong spin on it like a spiral throw (but I don't throw it).
Now I give it a slight (slow) nudge about an axis perpendicular to its fast spin.
It is now roatating about tow axes. No single-axis rotation can match this.
No.HackaB said:I see what you mean now. Can you explain how the angular momentum stays constant while the angular velocity vector moves around in a complicated way?
HackaB said:Can you explain how the angular momentum stays constant while the angular velocity vector moves around in a complicated way?
The wobbling of a football in flight is caused by the Magnus effect. This is a phenomenon in which the rotation of the ball creates a difference in air pressure on opposite sides, causing it to curve or wobble in the direction of the lower pressure.
The amount of wobbling in a football depends on several factors, including the speed of the throw, the spin of the ball, and the air density. A faster throw and higher spin will result in more wobble, while denser air (such as in colder temperatures) will have a greater impact on the ball's flight.
Yes, weather conditions can have a significant impact on the wobbling of a football. As mentioned before, air density plays a role, but other factors such as wind speed and direction can also affect the ball's flight and cause it to wobble more or less.
Professional football players have mastered the ability to control the wobble of a football through their throwing technique. By adjusting the angle and speed of the throw, as well as the spin on the ball, they can manipulate the Magnus effect and make the ball curve or wobble in a desired direction.
The Magnus effect is not limited to football. It is also used in other sports such as baseball, tennis, and golf. In fact, it is a fundamental principle in the design and performance of many sports equipment, including balls, rackets, and clubs.