Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Thomas Marschall on 29/04/2011 13:01:04

Title: What affects the rotation of a thrown tennis racquet?
Post by: Thomas Marschall on 29/04/2011 13:01:04

Thomas Marschall  asked the Naked Scientists:
   
Hi Chris,

If I throw a tennis racquet up in the air with the narrow side of the racquet facing up towards the sky, I can give it a 360 degrees turn and catch it again. But if I turn the racquet 90 degrees (as if to bounce a ball on top of the racquet) and try to do the same 360 degrees turn, the racquet does an extra 180 degrees turn around its long axis, so that the side of the racquet that had been facing down towards the ground now faces up.

Why does the tennis racquet turn differently when it is held with the narrow side facing up compared with when the string side faces up?

Thanks,

Thomas.

What do you think?

Title: What affects the rotation of a thrown tennis racquet?
Post by: yor_on on 30/04/2011 17:42:51

Huh?

You can make a racket describe a total 360 degrees turn any which way it seems to me? What am I missing here?

Title: What affects the rotation of a thrown tennis racquet?
Post by: yor_on on 30/04/2011 18:36:28

You mean that is is balanced differently relative the gravity? That the geometry of the racquet's mass distribution have a influence on the way it moves? You know, that's a pretty tricky question :)

Everything falling is accelerated the same in a vacuum right?
So will a balloon fall the same on the moon as a penny?

Yep.

Will the balloon, if I tape the penny on the above side relative gravity, then turn to let the penny be 'first' hitting the ground?

As proven by this highly stringent scientific experiment. Magic, or what? (http://www.schoolfreeware.com/Why_Does_Bread_Land_Butter_Side_Down_Page_1.html)

Why?

I'm pleased you asked, now we will see :)

Title: What affects the rotation of a thrown tennis racquet?
Post by: RD on 30/04/2011 18:47:35

Quote from: yor_on on 30/04/2011 17:42:51

Huh? ... What am I missing here?

the unintentional half twist about the ("long") axis of symmetry

I suspect the different grips are responsible :
the first instance the grip is cleanly (symmetrically) released so no rotation about the axis of symmetry.
In the second case the fingers catching one side of the handle on release of the frying pan grip adding rotation  about the axis of symmetry 

Title: What affects the rotation of a thrown tennis racquet?
Post by: yor_on on 30/04/2011 18:56:08

Yes, that seems like a very nice explanation RD :)

But then we come to the penny taped on the balloon.
Will it turn to let the penny be first meeting the ground?

And, will it do the same in a vacuum?