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I made a video of hovering ping pong ball with a blow dryer. I zoom into show the rate of spin changing as I angle the blow dryer. I then held a cardboard tube over the hovering ball so it would shoot up through the tube.
http://www.youtube.com/watch?v=PdT3ChUl-zk&fmt=22
Link to a good explanation for the hovering ball.
http://www.terrycolon.com/1features/ber.html
Regarding the cardboard tube, I recall reading about that a while back and didn't see any instance of this at youtube, which is why I made the video. I don't recall the physics involved. It's my guess that there is significant dynamic pressure below the ball, the tube captures a flow of air without allowing it to expand outwards (just upwards), and because the tube is just barely larger than the ball, it interferes with the air flow that would normally go around and above the ball, increasing the drag. Perhaps someone could offer a better explanation?
http://www.youtube.com/watch?v=PdT3ChUl-zk&fmt=22
Link to a good explanation for the hovering ball.
http://www.terrycolon.com/1features/ber.html
Regarding the cardboard tube, I recall reading about that a while back and didn't see any instance of this at youtube, which is why I made the video. I don't recall the physics involved. It's my guess that there is significant dynamic pressure below the ball, the tube captures a flow of air without allowing it to expand outwards (just upwards), and because the tube is just barely larger than the ball, it interferes with the air flow that would normally go around and above the ball, increasing the drag. Perhaps someone could offer a better explanation?