Do SketchyPhysics springs conform to Hooke's Law?
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I'm curious whether SketchyPhysics springs can be used to demonstrate the equi-distribution "points on a sphere" physics problem.
Can a network (or mesh) of springs, which each stretch or compress relative to forces applied by other attached springs, fluctuate until forces are spread evenly (a state of equilibrium)?
Physics demonstrations that I've seen (implemented in java) writhe, gradually dampening in smaller-and-smaller increments, until equilibrium is reached (or a close approximation thereof.)
Has Sketchyphysics spring physics advanced to the point of being able to model such a thing in SketchUp?
For a java example of this (i.e.; Thomson's Problem,) see...
http://physics.syr.edu/thomson/thomsonapplet.htm
I wouldn't expect to model an entire sphere, with hundreds of points - a simplified spring-mesh of a portion of the surface would do nicely.
NOTES:
In my initial trials, it appears that each attachment point would need the rotational capabilities of a ball-joint.Another example of a spring-mesh would be a trampoline membrane.
Apparently, a spring can pull (extension,) but can it push (compression)?
[ Taff }
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Some of what you asked went over my head. But here is some info on the springs in SketchyPhysics.
The "spring" joint is badly named. It is really a slider that acts springy. In fact I might remove it in a future version since its functionality is now part of the Piston joint (accel/damp).
I think I know how to do a proper spring now. Basically a springy slider with each end a ball joint. Ill add it in a future version.
In theory a trampoline type mesh is possible with the current version. But it would require a lot of fiddling to get it to work right.
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Dear Taff the Red Head,
A spring works in both compression and extension. The shock absorbers on your car are in compression. In fact they are always in compression as they are compressed by the weight of the car. A simple spring balance works with the spring in extension. The attached shows a simple schematic of a mass attached to two springs
I hope I have understood your post correctly and that I'm not trying to teach ....blah, blah.
Kind regards,
Bob -
Bob,
I am, indeed, familiar with spring physics (see my full profile.)
What I'm asking is whether the SketchyPhysics spring algorithm is implemented such that a SketchyPhysics spring acts as would a real-world spring.
Your description should, however, help other readers who aren't that famililar with the subject. For future SketchyPhysics spring implementation, it should be noted that a spring may not have a compression characteristic, if the spring is fully compressed to begin with (think of a typical screen-door spring.) In such a case, the coils of the spring are initially in contact with each other, and can compress no further (referred to as "coil-bound.") It may be useful to be able to specify a zero compression factor in a future SketchyPhysics release. (Or am I missing something that can be done now?)
Offhand, I can't think of an example of the reverse, where an unrestrained spring has a compression factor, but no extension factor (i.e.; you can compress it, but can't stretch it.)
Ain't physics fun? It looks as though SketchyPhysics is introducing physics to users who would otherwise completely avoid the subject. From a former physics teacher,...
...thank you, Chris!
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@taffgoch said:
Bob,
I am, indeed, familiar with spring physics (see my full profile.)
What I'm asking is whether the SketchyPhysics spring algorithm is implemented such that a SketchyPhysics spring acts as would a real-world spring.
Your description should, however, help other readers who aren't that famililar with the subject. For future SketchyPhysics spring implementation, it should be noted that a spring may not have a compression characteristic, if the spring is fully compressed to begin with (think of a typical screen-door spring.) In such a case, the coils of the spring are initially in contact with each other, and can compress no further (referred to as "coil-bound.") It may be useful to be able to specify a zero compression factor in a future SketchyPhysics release. (Or am I missing something that can be done now?)
Offhand, I can't think of an example of the reverse, where an unrestrained spring has a compression factor, but no extension factor (i.e.; you can compress it, but can't stretch it.)
Ain't physics fun? It looks as though SketchyPhysics is introducing physics to users who would otherwise completely avoid the subject. From a former physics teacher,...
...thank you, Chris!
Ok. I think I understand now. The screen door example made it clear. You are correct that the springs are not real springs. Both the "spring" and the slider/piston will just use a springy force to move the object to a desired position. As far as I know there isn't a way to make it behave as if there is a compression limit.
I guess a real spring would need the properties you suggest. And I think Newton should be able to handle it without too much problem. NewtonPlayground (http://www.saschawillems.de/?page_id=83) is also based on Newton. One of the examples named Newton Logo demonstrates springs as I hoped to implement it. Does that look like what you need?
You seem to know a lot about the subject. I don't suppose you know c++?
Chris
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Chris,
You may find this enlightening (and there's C source code, too.) The discussion includes algorithms for gravitation, viscous drag, and Hooke's spring law:
[Taff]
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Chris,
The textual description at the NewtonPlayground page, under "Springs," sounds about right - for extension, at least. There's no mention of compression characteristics, though. (It's basically the inverse, and it should be possible to implement it as such.)
Regarding your C++ question -- I stopped teaching myself new languages at Pascal and C. I'm an old-timer, and got tired of having to learn a new programming language every few years (Fortran, assembly, Basic, Pascal, C...) When C++ (Object-Oriented Programming) came out, I threw up my hands and gave up (although I'm currently considering the benefits of tackling java.)
I don't actually "know" that much about spring functions/calculations (but I know where to look in my library.) It's classical physics, so isn't as problematic as modern physics. I think the formulae for springs can probably be simplified to give a pretty good approximation of spring-function, without getting into calculus and differential equations. (Thank God!) Perhaps the Newton package already handles the implementations properly.
If a SketchyPhysics spring does not recoil (after stretching) or rebound (after compression,) thereby attempting to return to it's initial resting-state, then you're right that it wouldn't perform as 'true' spring would.
[Taff]
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Thanks for the quick response. I'm left wondering whether a spring has both a stretch AND a compression character to it. (That's basically what Hooke's Law describes.) I haven't looked at the Newton specifics, but would be surprised if it didn't cover this.
I'll play with it some more, to see what a SketchyPhysics spring can do. Ideally, I'd like to be able to set the 'resting' length of a spring, then the 'stretch' (pull)resistance and the 'compression' (push)resistance. A real spring has these three defining characteristics.
If it won't work as described above, perhaps you can add the features to your notes, for possible future implementation. Thanks for your comments. I'll take a look at piston settings, as well.
[Taff]
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