Re: Swing and momentum
- From: "Edward Green" <spamspamspam3@xxxxxxxxxxx>
- Date: 10 Jun 2006 09:39:45 -0700
Henning Makholm wrote:
Scripsit "Edward Green" <spamspamspam3@xxxxxxxxxxx>
Related playground question: how can a child standing in the middle of
a turntable, not touching anything else, make it rotate?
Friction in the bearing allows it to transmit a small torque to the
turntable. The child starts turning her torso with a torque that it
small enough to be countered by the turntable, then suddenly reverses
her rotation with a large torque in the opposite direction. The large
torque overcomes the friction and the turntable begins to rotate.
I don't think that the trick will work a second time once the
turntable does rotate. It depends on non-linearity of the friction.
Thank you for the answer, but not the answer I was looking for.
The child can make the turntable rotate even assuming a frictionless
bearing, even have a time averaged non-zero rate of rotation, although
he cannot put it into a steady state rotation. He does this by first
twisting one way then twisting back, having altered his moment of
inertia in the interim by extending or dropping his arms. The net
result will be a repeatable re-orientation of turntable + child. Of
course the net angular momentum, if zero at the first, must remain
zero: the point is, deformable bodies can reorient in the absence of
external torques.
You could say it was a trick question.
.
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