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Q & A: Magnus force on spinning objects

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Most recent answer: 10/22/2007
Q:
What is magnous effect in a spinning motion of an object?
- sridatri dutta (age 14)
TN , India
A:
A spinning object creates a kind of whirlpool of rotating air about itself. One one side of the object, the motion of the whirlpool will be in the same direction as the windstream that the object is exposed to. On this side the velocity will be increased. On the other side, the motion of the whirlpool is in the opposite direction of the windstream and the velocity will be decreased. The pressure in the air is reduced from atmospheric pressure by an amount proportional to the square of the velocity, so the pressure will be lower on one side than the other causing an unbalanced force at right angles to the wind. This is the Magnus Force.

This force is what is responsible for curveballs to curve in baseball, and affects noticeably the motions of ping-pong balls and golf balls and just about any ball in any sport that spins and moves. The Magnus force has also been used to push cylindrical "sails" (Jacques Cousteau had a boat with a big cylindrical sail on it)[ whoops- see correction about to be posted/ mbw]  and also some windmills have been made with spinning blades to take advantage of the Magnus effect.

(published on 10/22/2007)

Follow-Up #1: whoops

Q:
The cylindrical sail used by Jacque Cousteau is based on circulation control using a fan to suck air into holes on one side of the cylinder. This is not the "Magnus effect". So why do you describe it as the Magnus effect? The Magnus effect occurs with spinning objects, usually cylinders and balls.
- craftsman, inventor, writer (age 67)
Oakland, CA USA
A:
As nearly as I can tell from reading about that sail, you’re right. Thanks for the help!

Mike W.

On the other hand Anton Flettner, a German born aeronautical engineer, actually
built a true Magnus effect ship, complete with rotating cylinders.   The 'sucking' version
of Cousteau is equivalent, in a sense.
see: http://www.tecsoc.org/pubs/history/2002/may9.htm
LeeH

(published on 10/22/2007)

Follow-up on this answer.