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 However, they are a touch outdated.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 top and air underneath. Since then, there have been refinements so that less material can be used, cable stayed is better than suspension, which is better than cantilever etc etc.
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 middle of the Thames in a structurally efficient way, they're not going to be able to cross the river and might be trapped in South London forever.
The key is to make the bit of bridge you are currently using move. Movement is something bridge designers abandoned after their Tacoma narrows experiment demonstrated that the whole concept isn't their strong suit. However, within all that oscillation and harmonic stuff is a real solution. A pendulum.
We can take the Thames as our model river, which is about 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. From 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 22 seconds per one way trip, peaking at just under 40mph.
The weight at the bottom of the pendulum is our passenger car. Due to the nature of pendulums, as they approach the 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 with 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 you whizz to the other bank, then simply step off the other side. Done.
This idea is definitely fine and not halfbaked because for insurance purposes the bridge will demonstrated to have a total velocity of 0. Any accidents will, for legal purposes, just be people falling off a stationary object into the river. Not the bridge's fault. Boats of almost any height may pass freely by simply timing it right, they're used to tides, same principle.
Theoretically, pendulums are energy neutral, however, changes in passenger mass, air and bearing friction will all contribute to the pendulum slowing down. To combat this, 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 there on top of the tower, "mixed use" really helps planning applications.
* equal to one New York week-- bs0u0155, Dec 29 2017 https://en.wikipedi.../Transporter_bridge [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 continue.
// 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".
Just a suggestion ...-- 8th of 7, Dec 29 2017 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.-- MaxwellBuchanan, Dec 29 2017 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 ?-- 8th of 7, Dec 29 2017 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 platform.
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 water traffic-- Vernon, Dec 30 2017 //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.-- Voice, Dec 30 2017 Zeno was taking [Voice]'s side on this one, until Heracleitus sneaked up and pushed him in.-- pertinax, Dec 30 2017 If we're using mature clock technology, doesn't that imply simply digitising the whole river-crossing?-- pertinax, Dec 30 2017 // digitising //
Well, based on London's distinctive approach to hospitality, two fingers would be the most appropriate number.
// Technically I think they don't stop //
It's simple harmonic motion, so v = 0 for time delta-t, where delta-t -> 0.-- 8th of 7, Dec 30 2017 random, halfbakery