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Bridges are all well and good, I'm happy they exist.
However, they are a touch outdated. The general
principles were in place long before the Romans started
being so pleased with themselves for putting water on
and air underneath. Since then, there have been
refinements so that less material
can be used, cable
is better than suspension, which is better than cantilever
Shirley the ultimate progression of this idea is to remove
the bits of bridge that you're not currently on. A person
might be supported above a river by a single pole. While
this leaves our hapless subject suspended above the
of the Thames in a structurally efficient way, they're not
going to be able to cross the river and might be trapped
South London forever.
The key is to make the bit of bridge you are currently
move. Movement is something bridge designers
after their Tacoma narrows experiment demonstrated
the whole concept isn't their strong suit. However,
all that oscillation and harmonic stuff is a real solution.
We can take the Thames as our model river, which is
244m wide at Tower Bridge. Our pendulum should be
anchored at a point about 475m above the center-line of
the river. The anchor point can be done by bridge
engineers, since it doesn't move, they should be OK.
this point we hang our pendulum, which will swing about
15 degrees either side of center to just about kiss each
bank. The period comes to around 44 seconds or a zippy
seconds per one way trip, peaking at just under 40mph.
The weight at the bottom of the pendulum is our
car. Due to the nature of pendulums, as they approach
high point of their swing they slow down, stop for a
difficult to determine length of time, then increase in
speed in the opposite direction. This dwell at the end is
when passengers step on or off the platform. I reckon
a slightly sprung embarkation platform the pendulum will
be relativity stationary for at least a second*, more than
enough time to take one step forward. You hold on as
whizz to the other bank, then simply step off the other
This idea is definitely fine and not halfbaked because for
insurance purposes the bridge will demonstrated to have
total velocity of 0. Any accidents will, for legal purposes,
just be people falling off a stationary object into the
Not the bridge's fault. Boats of almost any height may
freely by simply timing it right, they're used to tides,
Theoretically, pendulums are energy neutral, however,
changes in passenger mass, air and bearing friction will
contribute to the pendulum slowing down. To combat
the mature technology of clockmaking simply needs a bit
of a scale up to power the whole apparatus. While you're
at it there's no reason not to put a big clock face up
on top of the tower, "mixed use" really helps planning
* equal to one New York week
[pocmloc, Dec 29 2017]
||// might be trapped in South London forever. //
||"The horror ... the horror ..."
// for insurance purposes the bridge will demonstrated to have a
total velocity of 0 //
||It will have an overall translational velocity of 0. Do please
// An accidents will, for legal purposes, just be people falling off a
stationary object into the river. Not the bridge's fault. //
||Defintely not. Sue them for negligence.
// the mature technology of clockmaking simply needs a bit of a
scale up //
||It may be apposite to draw to your attention that the technology of
solid-fuel reaction engines also falls into the category of "mature".
||This is an excellent idea, but you've built it upside down, [bs].
||Why have a pendulum suspended from an attachment point high above the river? This adds needlessly to the construction costs.
||What you want (and I have one somewhere - hang on, it may be behind those paint tins on the third shelf) is an inverted* pendulum. Basically, a springy rod with its bottom end embedded in the middle of the river, supporting the platform on which the river-crossing person stands.
||Energetically, it'll be no more or less effective than a non-inverted (ie, a verted) pendulum, but construction will be easier.
||*Speaking of inversions, I am just back from dinner with some friends. They had festive paper napkins, decorated with printed snowflakes and the words SNOW SNOW SNOW in various typefaces. Unfortunately, the napkins had all been laid upside-down with respect to the diners, so everybody had a napkin that read MONS MONS MONS.
||Thats nothing, I was in Asda a couple of days ago and a
staff member of the lady type wore a christmas
jumper which said
LE TITS NOW repeatedly around her circumference.
||That tells you all you need to know about the french - vulgar,
immodest, lacking in taste, and shamefully self-publicizing.
||Was she wearing any garments at all other than the jumper ?
||If the pendulum is stationary at each endpoint of its swing,
then it can be latched into place, giving plenty of time to
those who want to disembark or board the swinging
||Then unlatch it, let it swing to the other side, and latch it
again for unloading/reloading. I'm almost certain that the
added weight will not affect the distance the pendulum
swings. Note the latched platform is also out of the way of
||//stop for a difficult to determine length of time//
||Technically I think they don't stop. At any rate I know you can't prove they do stop.
||Zeno was taking [Voice]'s side on this one, until Heracleitus
sneaked up and pushed him in.
||If we're using mature clock technology, doesn't that imply simply
digitising the whole river-crossing?
||Well, based on London's distinctive approach to hospitality, two
fingers would be the most appropriate
// Technically I think they don't stop //
||It's simple harmonic motion, so v = 0 for time delta-t, where
delta-t -> 0.
||Two fingers? We usually suffice with just the one.