| MadSci Network: Physics |
Dear Mr. Bergeal
I've observed the strange behavior of the tennis racket last
week-end, and I repeated the experiment several times, ad-nauseaum. It's
really amazing! Funny, because I play tennis since I was six, having
thrown the racket in the air billions of times, never stopping to notice this
strange behavior.
Whatever occurs, it's an equilibrium problem of anti-symmetric
forces that actuate over the flat surface of the racket. Just pay attention
to the fact that there are no perfectly symmetric rackets. These forces
may be of aerodynamic or gravitational nature. There is no other possible
origin for the rotation, although it may happen to throw the racket in the
air with a little rotating residual effect, depending on the ability of the
"scientist".
Comparing the view of the racket from the vertical plane with the
view of the horizontal plane (following your convention), we notice that in
the last any difference between the right side and the left side is
amplified by the effect of an arm of 10 to 14 cm, while by the vertical
plane the same differences (as we assume the same racket) are amplified by
just one or half cm. So any imperfection in the natural symmetry of the
body is far more noticeable (more than 14 times higher) when we rotate the
racket by the horizontal plane instead of the vertical. If you think that
there is no difference between the sides of your racket, try this: hold
your racket using two pencils instead of your hands. Now you will notice
that if you place the pencils in the geometric center line of the racket,
it will almost certainly rotate alone. That's why even little differences of
weight distributions affects the rotating movement. This is what I mean
when I said "gravitational nature". Now, the same effect can be expected
by aerodynamic forces over the strings, that only happen when the racket is
thrown by the horizontal axis.
Well, that's all folks!
Fernando AriŅo Grau.
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