Rotation

[satsu]

Okay I have a question that I wish to be answered. Does the rotation on a ball change the direction of its destination, or is it the way in which you throw it that determines the destination of the ball? My deduction is that the rotation is the reason for it's destination, but on the other hand it could be the way you throw it, because the air has no traction in which the ball can grab onto therefore the way in which you force it is were it goes. So please answer my question: is it rotaion that effects the balls destination, or the way in which you throw it?

[thedarkshade]

I think it's rotation too. Because if you throw the ball rotating horizontally in a 180(scale) angle, then it's destination will change from it's primitive direction. But on the other hand you make that rotations possible by throwing it!

[insane_alien]

both. you are incorrectly assuming that air has no traction with the ball. a simple experiment to disprove this is to stick your head out of the window of a car going 70 mph. then try and tell me air has no traction.

 

if air had no traction the ball would follow a parabola regardless of spin. add in air resistance and you can affect direction because friction will be less on one side of the ball. wich is perfect conditions for a turn.

[iNow]

Have you ever thrown a baseball or played tennis? Spin is a major component of destination. As insane_alien said, the air does exert friction/resistance, and hence spin does matter. It might be different for a ball thrown in a vacuum (like outer space), but here within Earth's atmosphere there are curve balls which can be induced with rotation... itself caused by the way the ball was thrown.

[Bignose]

There are actually two different ways a fluid can interact with an object such as a ball to cause lift. There is a spin-induced lift and a shear-induced lift.

 

The spin induced lift is what has been talked about above, this is what happens when the object itself is spinning. The fluid flowing around the object is made to turn, thusly fluid that was approaching the object head-on is made to leave the object in a direction different than 180 degrees from where the fluid approached. Because the fluid doesn't leave in the exact opposite of the way it approached, there is net force. This is why a curve ball breaks in baseball, and a golf ball hangs in the air far longer than if it were dropped.

 

You can read more about this by looking up Magnus force, named after one of the first researchers to describe it. Wikipedia is a decent starting place: http://en.wikipedia.org/wiki/Magnus_effect

 

The other lift is caused by velocity gradients in the fluid motion. That is, if the fluid near the top of an object was moving faster than the fluid at the bottom of an object, then the top of the object would experience more force by the fluid than the bottom, and hence the object is caused to move in a non-square direction -- again called lift. A particle sitting on the a flat plate that has fluid flowing over it experiences this kind of lift -- the particle is lifted off the plate, leaves the boundary layer fluid flow near the plate and enters the bulk flow away from the plate.

 

However, in general, the spin induced lift is far more important than the shear induced lift. Both definitely exist, though, for any object moving through any fluid. So, to answer the OP's question, yes, the rotation of the ball definitely influences it's trajectory. Compare a slider and a screwball thrown at the same velocity. Both start in the same direction, but because they have opposite spins, they break in opposite directions.

[MrMongoose]

This might oversimplify it slightly, but using potential flow theory Kutta-Juhkowsky Lift is proportional to the circulation of the flow around the ball, so the ball will have a force acting on it in the direction of the cross product of its momentary direction and its momentary rotation vector