Important instrument on the
Richard James | Source: CNN
One day, he accidentally knocked down a newly developed spring. As a result, the spring fell from a pile of books on the bookshelf, but it was not at all embarrassing, but it drew an elegant arc like an acrobat. The same “somersault” level by level, and finally landed firmly on the ground.
James immediately thought that transforming this spring might make it “walk” smoothly.
He tried various steel wires, adjusted the shape of the spring, and finally got a spring that could “automatically go downstairs”.
Picture source: MUSICOM PRODUCTIONS
He shared this joy with the neighbor’s children: put the top of the spring on one step, and the end on the next step, then loosen the top, the spring immediately turned somersaults unhurriedly , “Go” down the stairs!
The children’s keen interest in this novelty made James and his wife Betty see its commercial value. Betty named it “Slinky”, which was later translated as “Slinky”, which vividly shows when it goes down the stairs. Looks like.
Betty, who is quite business-minded, began to promote it in TV commercials, and in just one and a half hours, 400 Slinkys were sold out.
The original Slinky is made of steel wire
Since then, Slinky once swept the world and became a popular toy. From 1945 to 2005, it sold more than 300 million.
James applied for a patent for Slinky. It is made in a very simple way. Thin steel wires more than 20 meters long are rolled flat and tightly compressed and wound together.
Slinky’s patented design | Source: U.S. Patent 2,415,012
Later, a colorful plastic model was added, which is the “rainbow circle” we often see now.
The biggest difference between it and ordinary rigid springs is that Slinky is a soft spring, which has reached the tightest compression state in its natural state. The ring is close to each other and can only be stretched and cannot be contracted further. . It is soft and loose, and can be greatly twisted into various shapes, and finally can be restored to its original shape.
This characteristic of the soft spring gives it some wonderful performance.
Going down the stairs is the developer’s default gameplay, and then some players have played new tricks ingeniously.
For example, let Slinky go on a treadmill to keep fit and compare speed with an escalator.
Slinky: Are you trying to exhaust me?
Physics teachers also like to use it as teaching aids to introduce various physics principles.
The simple structure of Slinky, the physics behind it is not simple, even physicists are fascinated by it.
The following stairs are taken as an example, the following is the force analysis when Slinky is placed on platforms with different heights(For detailed analysis, please refer to References).
Source: Liu Yanzhu, Mechanics and Practice, 1996, 18(3): 70-71.
When the left side of Slinky is higher than the right side (as shown in the picture above), the left half of the support is (FN1)’s point of action is more offset from the upper platform edge than the right half of FN2’s point of action (ie The y-axis in the figure).
When the height difference between both sides is sufficientWhen it is large, the point of application of the left support force will extend beyond the edge of the upper platform. As a result, as soon as the person let go, Slinky’s left side would lose the support of the platform, jump up and flip to the right.
Once it jumps over the central axis, it will drop a graceful “lower waist” under the action of gravity, fall onto the next step, and start a new cycle. As a result, Slinky showed the stunt of “down the stairs”.
In this process, Slinky seems to have no power source, but in fact it is constantly transforming gravitational potential energy into falling kinetic energy. In each cycle, elastic potential energy is also participating in energy conversion.
As long as the state of each staircase is consistent, Slinky will repeat this process periodically, as if he was walking down the stairs.
Picture source: JeepersMedia
Slinky has another special skill, which looks quite weird.
Let’s do a physics problem first: hold one end of Slinky in one hand, let it sag naturally, and then let go. How will Slinky move at this time?
Fall together? Or does the top fall and the bottom bounce?
Since I said weird, the above two intuitive answers are definitely wrong.
The answer is that the top will fall quickly until it retracts to the bottom, and the bottom does not begin to fall.
Time-lapse photography of Slinky drop | Source: Veritasium
Let’s also do the force analysis first. In the beginning, the suspended Slinky was pulled by the hand and balanced with the downward gravity.
Analyze the force of each ring separately. The uppermost ring has to bear the weight of the entire Slinky, and at the same time have the pulling force directly provided by the hand to balance it.
The lower the ring, the lower the weight it has to bear, and the lower the elastic force applied to it by the upper ring. When it comes to the last ring, the elastic force it receives will only offset its own gravity. .
Therefore, the entire Slinky stretches less and less from the top to the bottom, and the ring becomes denser.
At the moment of letting go, the upper ring suddenly lost the pull of his hand and fell quickly.
You should know after playing the rainbow circle that when you put the rainbow circle loosely on the table and only move one end gently, the other end can be almost unaffected, unlike a rigid spring that moves the whole body.
Due to this nature, when the force on the top ring changes, the ring below it will not immediately follow this change. Since the deformation is still there, the elastic force received by the lower ring will not disappear immediately, and the gravity can still be balanced until the upper ring presses against itself. According to Hooke’s law, as the ring falls, the stretching distance decreases and the elasticity decreases, and gravity prevails.
The process of falling rainbow circle | Source: Cross, R. C., & Wheatland, M. S. (2012)
From the overall point of view, the process of Slinky’s top-down compression is a mechanical wave transmission process. Only when the shock wave is transmitted, the balance of forces on the bottom ring will be destroyed. , And then fall.
So the peculiar phenomenon that the bottom of Slinky hangs in the air occurs. For a standard size Slinky(pictured above), the hovering time is about 0.3 Seconds, you have to use time-lapse photography to see clearly. The hovering time depends on the speed of the mechanical wave transmission, which is related to the length, material and other factors of Slinky.
When Newton and Galileo saw this phenomenon, wouldn’t the coffin board move?
No need. Although the acceleration of each part of Slinky is different at the beginning, the overall center of mass is still falling at the acceleration of gravity (g).
Kolkowitz, a student at Stanford University, published a paper on this phenomenon. He found that the same Slinky on the earth, the moon, and Mars, the bottom of the hovering time is the same, that is, this A phenomenon is not affected by the strength of the gravitational field.
Bill Unruh, a professor of astrophysics at Columbia University(Bill Unruh) was very interested in this, and he also specially created the Slinky drop The physical model, he lamented: In a very simple system, there is also a wealth of physics knowledge.
Sure enough, both inventors and scientists have fun.
They all say “playing things to lose their minds”, isn’t this very inspirational?
Picture source: Gifmania
Reference: https://shimo.im/docs/RRKvqTdWjVqPkQQr
This article is from WeChat public account:Bring science home (ID: steamforkids) span> , author: Mirror