h a l f b a k e r y
This is what happens when one confuses "random" with "profound."
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The Springy Tire Swing is constructed in the same fashion as a traditional tire swing except that it has an elastic rope instead of the traditional inelastic rope.The length and stretchyness of the rope are such that while a rider is seated in the tire the frequency of bouncing is approximately two time
as fast as the frequency of swinging. When the rider causes the Springy Tire Swing to bounce up and down, the Springy Tire Swing will begin to swing back and forth on it's own. The swinging motion will quickly become very large, and the combination of the bouncing motion with the swinging motion will guarantee lots of laughs and broken arms.
[Disclaimer: If made properly, this device WILL work and could be incredibly dangerous.]
Works with squirrels apparently.
The motion here reminds me of a bird feeder that repels squirrels. [skinflaps, Feb 20 2009]
Spring Pendulum Paper
One of many papers on this subject [GravesWaves, Feb 20 2009]
The reason the springy tire swing bounces back and forth on its own. [GravesWaves, Feb 20 2009]
this is a cartoon movie of a system that is very similar but not as crazy [GravesWaves, Feb 23 2009]
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||The difficulty I see is that the weight of the passenger will affect whether it works; how could this be compensated for?
you are correct that the weight of the passenger effects whether or not it works. The condition for it working is:
||where F is the weight of the rider (+tire), k is the spring constant of the elastic rope, and L is the length of the rope. k can be adjusted by having a bundle of bungees where some of the bungees can be disengaged.
The principal behind the Springy Tire Spring is parametric resonance. The condition 3F=kL creates an instability so that up and down motion will create side to side motion. The Springy Tire Spring is not just a swing that also bounces. I doubt that the thing you saw was set up for parametric resonance because a heavyduty spring would likely produce a bouncing frequency which is more than twice the pendulum frequency. The thing you saw also has the problem that vincevincevince mentioned. Fishbone me if you will, but please reconsider your declaration of bakedness.
||It seems completely counter-intuitive to me that weight should matter as long as the springiness of the rope isn't bottomed out. I sort of get it after the links, but it still feels wrong.
||Adjusting the tire swing for different weight riders is a detail. I already suggested one of many solutions to that problem.
||Check out the link for the Java applet. This applet is similar to the spingy tire spring except that it is not driven. The motion of the springy tire spring will be more extreme