wrote:
In article <4217FAA8.48594052@hate.spam.net>, Uncle Al <UncleAl0@hate.spam.net> writes:
carloman3 wrote:
how does a swing work why by moving your feet you get thrust?
i have no idea of the answer
Parametric amplifier. You can't pump up a stationary swing.
Yes, you can. It's not easy, but it can be done. Grip the chain with
your hands. Kick forward and back with your legs. Notice fore and
aft pressure of hands on chain. Notice that chain is displaced
fore and aft. This means that there is unbalanced horizontal force
from the chains onto you. This can be exploited to drive the swing.
Concur. Been there, did that. But to actually generate horizontal
displacement of your CG (start swinging) you have to first displace it
vertically (hunch down then straighten up) slightly out of phase with
the kicking. Very subtle point.
One of the hard parts is that without a noticible swing arc, it's
hard to time your rocking back and forth on the seat to match the
period of the swing. This is key. Randomly rocking back and forth
doesn't buy you anything. It's a driven harmonic oscillator. You
need to drive it at the proper frequency.
I think of this as "angular pumping" -- exploiting the angular
momentum of the rider.
Yep. Note that angular momentum is conserved; we're converting two
degrees of total zero linear momentum (cyclically changing center of
mass height while cyclically kicking) to total zero angular momentum
(swinging back and forth).
The other hard part is that without a significant swing arc,
raising and lowering the body's CG does not pump the swing.
Which is why you have to kick too. Gotta exploit both linear degrees.
It is this raising and lowering of CG that has the best utility
for pumping the swing at high energy levels.
I think of this as "linear pumping" -- exploiting the linear
momentum of the rider.
Presumably, it is "linear pumping" that Uncle Al was thinking
of when he made his claim.
His point is that with nothing to amplify (no initial nonzero
horizontal displacement) you can't start swinging which is strictly
correct in the abstract, but I don't think he's ever actually tried it.
OTOH I also think that real-world swings allow us to "cheat" because
the frames aren't ideally rigid, which allows the pivot point to move
the least little bit while starting. I don't think you could get started
on a swing hung from a truly rigid frame no matter how you kicked,
bobbed, or otherwise thrashed around.
ISTM that a swing is more like a paramp with the output looped back
to the input, and chaotic rather than linear circuit elements; what you
get out depends on how you pump it and the constants of the circuit
elements. It's chaotic because if you sit still after getting going you
swing like a pendulum, but you can pump your way slightly off the
attractor. The fun part is that you can pump your way onto the attractor
from a standstill in the first place.
I learned on an old style swing with 20 foot chains and a wooden seat
in my back yard. The optimal way to pump that one was from a standing
position, going into a crouch at the top of the swing and coming out
of that crouch at bottom dead center both ways. Linear pumping.
Long chains, high amplitudes and a standing posture favor
linear pumping.
Short chains, low amplitudes and and a sitting posture favor angular
pumping.
Well, standing up you can't kick. ;>)
And, of course, there's nothing that says that you shouldn't pump
both ways. With practice, most of us do both without even thinking
about it.
Yep. Kids learn a lot of real-world physics that just don't explain
well after we've learned abstractions and idealizations.
Mark L. Fergerson
.
