Re: Swing and momentum
- From: briggs@xxxxxxxxxxxxxxxxx
- Date: 5 Jun 2006 10:28:15 -0500
In article <87r723elw0.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>, Henning Makholm <henning@xxxxxxxxxxx> writes:
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.
If you do the experiment on a swivel chair rather than a playground
turntable, the possibility of multiple acceleration cycles is rather
distinct. I've done that experiment many times.
As a practical matter, most playground turntables acquire a non-negligible
tilt. If the bearings are good, it is possible to stand in place just
off-center, walking uphill as on a treadmill while the turntable
accelerates beneath you.
Having done the experiment in my youth, I must say that it's easier to
start the rotation than to reverse it, once started.
No broken bones. But it hurt rather a lot.
.
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