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Listen to ExperimentFlying Tubes
Make an old kitchen,or toilet, roll tube fly and find out what it has to do with David Beckham's free kicks.
What you need
| A kitchen roll tube |
| 2-3m of narrow knicker elastic |
| Strong tape |
| A table in a room with nothing breakable or a still day outside. |
What to Do
Attach the elastic to the middle of one of the long sides of your table so it stretches about 2/3 of the length of your table.
Point this side at a lot of space (7-8m at least)
roll the elastic stretched onto the centre of your tube until it is on the far side of the table to where the elastic is attached. This works best if you put 1-2 turns on the tube while the elastic is quite loose, then stretch the elastic and add another 2-3 turns.
Let go!
What may Happen
With any luck the tube will lift up as it flies through the air instead of just falling down. It will probably take some experimentation with how tight and how much to roll up the elastic, but it looks cool when it works!
What is going on?
If the tube is not moving too fast the air going past the tube will tend to stick to it (the Coanda effect). If it is not spinning the air will stick to it symmetrically so leave the back of the tube symmetrically.
If the tube is spinning however the air will stick to the part of the tube moving with the air (the top) as opposed to the part moving against it (the bottom) where it tends to separate from the tube earlier. This means that overall the air passing the tube ends up being pushed downward, so because for every action there is an equal and opposite reaction the tube gets pushed upwards, and flies.
If you fire the tube too fast the air will separate earlier on both sides and there will be less of an effect.
Real World
David Beckham's kicks swerve in the air, he gives the ball spin when he kicks it so it will generate lift, the direction of this lift depends on which way he spins the ball. They tend to swerve late because he judges the speed exactly right so the air streams start off seperating from the ball and then reattaches to it late on in the flight producing a swerve.
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If the ball is going too fast the air streams separate from it much earlier creating a turbulent region behind it and not much lift to swerve it. | As the ball slows down the air stream reattaches to the ball. Where the air is moving with the ball's spin it stays attached for longer than where it isn't so the air gets deflected much more and so the ball swerves more. |
Tennis players also use top and back spin extensively, because they need to get the ball over the net and still stay in the court. If they hit the ball too fast they will overshoot, but if they use topspin the ball pushes the air upwards, so it gets pushed downwards, staying in the court.
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