Posted on 10/04/2012 4:04:07 PM PDT by jwsea55
Watching this video, I have to ask, do we really understand gravity? When we release an object from our hands, it falls. Right? Not always? Or at least, not right away?
Veritasium has put some pretty cool videos to explain how science and physics work. They have been working with slinkies on a number of videos (and you thought your kid didn’t have any potential at 3 years old), this video seems to capture the essences of their work.
So listening to a couple of science guys explain this, does this give one a solid enough understanding why that darn bottom of the slinky doesn’t move? OK, I get the propagating communication of information thing, and that gravity has a constant force on the center of gravity but when I step back from all that, why doesn’t that slinky’s bottom move? Jim Caldwell
Thanks
If you had different tensions/thickness of steel at the oppsite ends of the slinky, I wonder if that would produce humorous results upon release?
I do have a question on the The extension of the slinky is entirely due to ITS OWN mass.
Is this still a correct statement when you lay the slinky on its side (Xing out old slinkies will want to flop on their ends)?
Thanks again for the great post.
I thought there was an interesting story behind the slinky...It is. The story is fitting of the slinky itself.
In 1943, Richard James was a naval engineer trying to develop a meter designed to monitor horsepower on naval battleships. Richard was working with tension springs when one of the springs fell to the ground. He saw how the spring kept moving after it hit the ground and an idea for a toy was born.
Traespiral
Richard James told his wife Betty, "I think I can make a toy out of this" and then spent the next two years figuring out the best steel gauge and coil to use for the toy. Betty James found a name for the new toy after discovering in the dictionary that the word "Slinky" is a Swedish word meaning traespiral - sleek or sinuous.
Slinky debuted at Gimbel's Department Store in Philadelphia, Pennsylvania during the 1945 Christmas season and then at the 1946 American Toy Fair. Richard nervous at the first demonstration of his toy convinced a friend to attend and buy the first Slinky. However, this turned out to be unnecessary as 400 were sold during the 90 minute Gimbel demonstration.
James Spring & Wire Company
Richard James and Betty James founded James Spring & Wire Company (renamed James Industries) with $500 dollars and began production. Today, all Slinkys are made in Hollidaysburg, Pennsylvania using the original equipment designed and engineered by Richard James. Each one is made from 80 feet of wire and over a quarter billion Slinkys have been sold worldwide.
Richard James - Cult Involvement
Around 1960, Richard James suffered from a mid-life crisis and left his wife, six children, and the Slinky Empire to join a Bolivian religious order/cult. Betty James took over as CEO of James Industries and rescued the company from the debts left by her husband's generosity to his religion. She moved the company to its current Hollidaysburg location from Philadelphia and began an active advertising campaign complete with the famous Slinky jingle. Richard James died in 1974.
Betty James Continues
Betty James also replaced the original material of blue-black Swedish steel with silver colored American metal. She added other toys to the line: Slinky Jr., Plastic Slinky, Slinky Dog, Slinky Pets, Crazy Eyes (glasses with Slinky-extended fake eyeballs) and Neon Slinky. The line was sold in 1998 to Poof Toys. Betty James was inducted into the Toy Industry Hall of Fame in 2001.
http://inventors.about.com/od/sstartinventions/a/slinky.htm
I forgot to add you to post 45. History of the slinky...fitting of the slinky. Checkout some of the post above that. Descriptions on how that thing works.
W. G. Unruh's succinct paper on the matter here.
When a Slinky is dropped, the bottom of the Slinky remains motionless as the top collapses towards it, making it appear to the observer as though the Slinky is levitating. By considering the Slinky as a tightly wound, pretensioned spring, the static equilibrium of a hanging Slinky was solved for using Hooke's law (Equation 1). This result was used to measure the spring constant of an actual metal Slinky. The motion of the Slinky after it is released at time t=0 was then solved for to derive an expression for the time over which the bottom of the Slinky remains motionless and the Slinky appears to levitate (Equation 7). This expression gave a value of t = 0.29 ± 0.05 seconds for the Slinky used in the experiments, which matches up very well with the experimentally measured value of t = 0.4 ± 0.1 seconds
All of the examples I talked about pretty much presume that the axis of the slinky, which is vertical when the slinky is suspended in the video, remains vertical and straight at all times. A real slinky gets its extension/compression spring constant and its torsional spring constant from the nature of the material making up the coils.
Bending the slinky so that its axis is not a straight line, which is what I think you are suggesting, would be a demonstration of a degree of freedom which is not taken into account whatever in my description. Such bending introduces additional ways in which the slinky can interact with a gravitational field or store energy by changing shape.
If you lay the slinky on its side on a horizontal surface such that the axis is straight, and ignoring friction with the surface, the slinky should seek out its neutral position of minimal energy. For a real slinky, this may in fact be where the sides of adjacent coils are touching. There would be no freedom to compress the slinky any further, barring deformation of the coils.
Without friction with the surface, the slinky would exhibit a spring constant describing the amount of force required to separate the coils a particular distance, the distance being proportional to the force.
Sitting on its side, the slinky, though nominally of circular shaped coils, would undergo a very slight deviation from that circular shape due to gravity, similar to the non-uniform extension that I previously described.
Each circular coil of the slinky is supported at a point on the bottom of the coil. This section of each coil supports the entire coil. The parts of the coil halfway from the point of contact to the upper edge of the coil will be supporting only the upper half of the coil. Each coil will then VERY SLIGHTLY (due to the stiffness of the coil) change shape to reflect the load on it. To a first approximation, the circular coil will become slightly oval due to this loading. I think the curvature at the bottom of the coil will be less than the curvature at the top but of this I'm not certain.
Did I understand your question?
It has been a few years away from this type of stuff (in a totally different area using math). The most important thing one can remember when going through grad school, "You CAN'T divide by ZERO!"
Sounds like you could teach half a year's physics class on thing. It really is one amazing device for storing and releasing energy.
I am sure I am not the only one who appreciates your time and thoughts! This has been a very entertaining and informative thread.
This thread exhibits one of the reasons why FR is a unique website. ... What an interesting discussion this has been!
I watched closely, again, and listened, again, to what this professor tried to explain. (Forgive me last time I just tried to explain in plain English why it appears the bottom of the slinky doesn’t move and didn’t study this video closely) The “signal delay” to get from the top of the slinky to the bottom part of the slinky is nonsense. He is attempting to assign some magical property to a slinky. He conveniently ignored what happened when you held the top of the slinky at a fixed height and allowed the slinky to extend under the force of gravity. All you see in this video is the end result of the stretched out slinky dropped from the top after extension so it only appears the bottom doesn’t move.
The easiest I can explain it is every coil on the slinky has fallen as much as it possibly can with spring tension exactly matching gravity. The coiling and twisting properties he tries to explain as “signal delay” is silly. All those “signals” are contained in the tension / torsion of the spring and are stored in all the various kinetic fashions when the spring (slinky) was stretched out. This “storage effect can be seen by the top coils being a greater distance then the bottom coils. Each of those coils are storing energy. If you were to watch the drop that stretched out the slinky in slow motion (which was not provided) you will see the twists and turns as they occured.
P.S. I design aircraft flight control systems. You can spend a lifetime on spring mechanics and still not understand the physics involved. I suspect this professor is demonstrating a case of “protect the hypothesis and protect the model” as is so common as demonstrated in global warming models. He is a lot smarter than me indeed but when he fails to show and explain what happened to that slinky when he uncoiled it (dropped it but held on to the top) I am very suspect of his explanation. In fact, I comfortably call his conclusions bogus. When he and the presesnter start talking about a lead slinky acting differently and whatnot he is engaging in nonsense. A spring is a spring is a spring and they are all subject to the same physics. The exact same thing will happen with a lead slinky, a plastic slinky, or an Obama unicorn fart slinky.
This is nothing more mysterious than a stretched out compression spring and gravity going on. No magic, no delayed signal, no “gravity is faster than light,” no changing center of mass. It is simple spring mechanics and I don’t dare go to the math equations involved.
>Reference for anyone wishing to mathematically challenge me -— Machinery Handbook 26 Edition, pg 285-332
>Hooke’s Law
>engineersedge(dot)com
>K factor
>http://www.newton.dep.anl.gov/askasci/eng99/eng99245.htm
I watched closely, again, and listened, again, to what this professor tried to explain. (Forgive me last time I just tried to explain in plain English why it appears the bottom of the slinky doesn’t move and didn’t study this video closely) The “signal delay” to get from the top of the slinky to the bottom part of the slinky is nonsense. He is attempting to assign some magical property to a slinky. He conveniently ignored what happened when you held the top of the slinky at a fixed height and allowed the slinky to extend under the force of gravity. All you see in this video is the end result of the stretched out slinky dropped from the top after extension so it only appears the bottom doesn’t move.
The easiest I can explain it is every coil on the slinky has fallen as much as it possibly can with spring tension exactly matching gravity. The coiling and twisting properties he tries to explain as “signal delay” is silly. All those “signals” are contained in the tension / torsion of the spring and are stored in all the various kinetic fashions when the spring (slinky) was stretched out. This “storage effect can be seen by the top coils being a greater distance then the bottom coils. Each of those coils are storing energy. If you were to watch the drop that stretched out the slinky in slow motion (which was not provided) you will see the twists and turns as they occured.
P.S. I design aircraft flight control systems. You can spend a lifetime on spring mechanics and still not understand the physics involved. I suspect this professor is demonstrating a case of “protect the hypothesis and protect the model” as is so common as demonstrated in global warming models. He is a lot smarter than me indeed but when he fails to show and explain what happened to that slinky when he uncoiled it (dropped it but held on to the top) I am very suspect of his explanation. In fact, I comfortably call his conclusions bogus. When he and the presesnter start talking about a lead slinky acting differently and whatnot he is engaging in nonsense. A spring is a spring is a spring and they are all subject to the same physics. The exact same thing will happen with a lead slinky, a plastic slinky, or an Obama unicorn fart slinky.
This is nothing more mysterious than a stretched out compression spring and gravity going on. No magic, no delayed signal, no “gravity is faster than light,” no changing center of mass. It is simple spring mechanics and I don’t dare go to the math equations involved.
>Reference for anyone wishing to mathematically challenge me -— Machinery Handbook 26 Edition, pg 285-332
>Hooke’s Law
>engineersedge(dot)com
>K factor
>http://www.newton.dep.anl.gov/askasci/eng99/eng99245.htm
Hmmm...
I worked with a guy years ago who was taking simultaneous measurements of voltage and current and given more time I think he was going to re-discover Ohm's Law. He was very fascinated by the proportionality which he had discovered.
I think this researcher has chosen poor words to describe what I think is happening.
Looking at the video there does appear to be a transition in the behavior of the spring when the coils close completely. The transition between the region where the coils are completely closed, and the region where they are not, does seem to travel downward along the length of the spring.
When this transition finally reaches the bottom of the spring, the spring force accelerating the bottom of the spring toward the center-of-gravity of the spring disappears. The bottom of the spring doesn't begin dropping because it receives a signal, it drops because it lacks the force which has held it in place.
Aside from some torsional effects, it does appear that the region of the spring below the collapsed part, is stationary.
I'm still struggling to imagine how such a non-uniformly extended spring would behave without a gravitational field.
LOL! That’s what mine always ended up looking like.
Many thanks! Mind = blown.
Gravity works with the compression of the spring. I wonder if this changes the equations of gravity?
Why does the force of gravity build up? Why is it not instant?
P.S. The spring weight is the same compressed and uncompressed.
On a side note - the other video at the bottom of the page is hilarious! “Don’t sleep on Barry O!”
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