Nobody wrote:Thanks @ Mark for your search function help and @ MrVibrating for pointing out that scissor-jack acceleration phenomena:
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Mr Vibrating wrote:
Final thought on scissorjacks - the only significant aspect i've found concerns the relative input-to-output
accelerations - as more scissor pairs are added to the jack's length, the relative I/O accelerations increase
linearly. This is why a long jack seems to open and close surprisingly rapidly when the handles are
pumped (like a linear whiplash effect). However i doubt this is the useful property Bessler hints at, and the
power requirements remain a straightforward function of mass, distance and time.
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Maybe the different and rapid acceleration/movement on one side, resulting also high forces of inertia shouldn't
be ignored, or can be used in a sensible way?
Bessler AP part 1 page 82:
Und wenn ein Pfund ein Viertel fällt /
Es vier Pfund hoch vier Viertel schnellt.
There is no better German word than the word "schnellt" that would describe that rapid movement/acceleration
on one side of a scissor-jack or stork´s bill.
For instance you could say:
A fish "schnellt" out of the water or an arrow "schnellt" through the air.
Best regards,
Nobody
Interesting thoughts, i'd noticed one or two clues perhaps hinting at speed but hadn't given the matter too much thought...
Still, just to clarify the jack's speed dynamic, it works like this:
- suppose we have a single scissor 'X'; it only has angular speed, not linear
- however for a double scissor 'XX', both have the same angular speed, but the 2nd also has some linear acceleration
- for N scissors, all have the same angular speed but the terminal scissor has linear speed roughly equal to N times the radius of one scissor times the angular speed (ie. how quickly the handles are operated)
This assumes each scissor is identical and there's no play in the bearings.
As i say though, it seems all that really matters is the conventional mass displacements. As beguiling as the speed issue is.. ie. if inertia was no issue a scissorjack would be an easy way to exceed lightspeed...
However, as a thought experiment, suppose we had a scissorjack with so many sections that if the handles are closed in 1 second, the terminal end will reach lightspeed. The jack itself can be notionally massless, however suppose there
is a small mass at the terminal end, that's going to be accelerated.
What would happen when we try to operate the handles? Even if we had infinte energy to pump the handles with, we'd never be able to open or close the jack in 1 second.
To take it a step further, suppose our jack had infinitely many sections... thus, any motion at all, however slight, applied to the handles will inevitably result in a superluminal acceleration at some point further along the jack, hence the handles would be inoperable, even with infinite input energy.
This illustrates that with regards to speed, at least, the snappiness of a scissorjack is a red herring... we might as well remove it from the thought experiment entirely, leaving only the prospective masses to be accelerated, since this is really what defines the possibilites, not the jack mechanism. It might as well be replaced with pulleys and tackle or any analogous mechanism... it's just linear leverage.
However in light of your point i intend to revisit this when i get time, perhaps i've overlooked something after all..
ETA: was playing with 'inertial pumping' last year, the attached sim uses a scissor section to try to coordinate an effective pump, but to no avail.. it just conserves input RKE. However it didn't occur to me to try multiple sections, ie. with longer jacks.. something else to try when i get the chance...
