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In lugnet.technic, Paul Davidson writes:
> A counterweight doesn't change the speed at which something falls, does it?
> It may reduce the apparent weight of the main structure, but objects of
> different weights fall equally fast.
Not entirely true. Your thinking of the "what falls faster, a tonne of lead or
a tonne of feathers" question (or - "what's heavier, a tonne of lead or a
tonne of feathers") which is actually more complicated than it seems.
The thing with a counterwight is that it contains potential energy. From what
I understand you want to control the decent of an object. If you have a
counterweight that is slightly lighter than the main object (ignoring
mechanical loss in the pulley system, etc), gravity will take over the descent
of the object and it will gradually accelerate downwards (realising its
potential, turning it into kinetic energy). Eventually it will reach a
terminal velocity (I'm only ignoring mechanical loss, otherwise it will
accelerate to infinity). Both the counterwight and the main object exert a
downward force (acceleration x mass) on the system, but the net force is down
for the main object, and always increasing as acceleration has the per second
per second factor.
In the wonderful world of reality you have mechanical losses due primarily to
friction in this case. There will be a point where the friction in the system
will balance out the net acceleration and the system will reach a constant
velocity.
With LEGO this will be difficult to achieve IMO as you are dealing with small
masses, which makes the system hard to fine-tune. A small difference in the
counterweight/main object weight will produce a slow decent, but it takes a
while to get going. The greater the weight difference the greater the final
velocity and the greater the acceleration. Inducing friction into the pulley
may help with the fine-tuning, either with elastic bands or using the technic
axle stoppers (the latter is a "cheap" way of doing it, but needs constant re-
adjusting as the stoppers work loose).
I recently made a (very bad) Star Wars shuttle with moving wings. I used the
fly-wheel system to control the descent of the wings. Worked pretty well
actually. I like the idea of the scissor system, perhaps combining it with a
pneumatic cylinder on the bottom arm to overcome the short cylinder travel of
the pneumatics, and as suggested crimp the outlet hose to control the descent.
I hope that makes sense. I guess I'm saying that all objects will eventually
fall at the same velocity (terminal velocity, and lets keep it to planet
Earth's gravity), but there are ways and means to control the rate at which
this velocity is reached, and ways to control and reduce it. Hard to explain
in a short amount of time, and I hope I'm not teaching anyone to suck eggs ;-)
Pete Callaway
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