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"Snap!"
In the Marvel movie "The Avengers", the villain boss Thanos can make half of the human race on the planet disappear with a snap of a finger.
The infinity glove, encrusted with gems, is full of mysterious powers.
However, fading the fantasy color of the movie, Thanos wearing metal gloves, can really hit the ring finger?
Recently, a scientific research team from the Georgia Institute of Technology and its collaborators gave the answer - the metal infinity glove may just make Thanos unable to hit this ring finger!
Inspired by this problem, they carefully studied the physics behind the "snap finger" and found that the human body produces the highest acceleration currently found when the finger is snapped.
The researchers say the results could help improve prosthetic simulation design, especially in mimicking the broad range of functions of the human hand, and may also improve the maneuverability of various robotic systems.
The research paper, titled "The ultrafast snap of a finger is mediated by skin friction," has been published in the scientific journal Journal of The Royal Society Interface.
Why?
In this study, Saad Bhamla, an assistant professor at Georgia Tech's School of Chemical and Biomolecular Engineering, formed a group dedicated to "snapping fingers."
When they saw Thanos wearing metal gloves and snapping his fingers in the movie, they questioned:
When snapping fingers with bare hands, it may be that skin friction plays an important role, and due to the frictional properties of metal gloves, Thanos may not be able to hit the fingers at all.
First, they conducted experiments and analyzed various finger-snapping situations using high-speed imaging, automated image processing, and dynamic force sensors. Subjects began to snap their fingers when wearing rubber finger sleeves, metal finger sleeves, and lubricant respectively, and tested the frictional effects, rotational speed, and rotational acceleration in each case.
Figure | Friction and velocity comparison when fingers are covered with lubricated rubber finger sleeves (red), nitrile rubber (green), latex rubber (purple) and metal finger sleeves (blue) (Source: Thesis)
The results showed that the minimum friction measured while wearing a metal finger sleeve was the same as the maximum rotational speed when the bare hand was.
That is to say, Thanos wearing his infinity gloves can't beat his fingers at all!
They believe that the potential reason may be that after the metal casing covers the fingertip, the contact area of the finger is reduced when the finger is tapped, resulting in a decrease in acceleration.
"The compression of the skin makes the system more fault-tolerant," says Challita, one of the paper's authors, "reducing the compressibility and friction of the skin will make it harder for you to build enough strength in your fingers to snap your fingers."
So, another question arises, is it easier to snap your fingers the bigger the friction? Actually, the answer is no.
As can be seen from the image above, increasing the friction between the fingertips and the rubber cover will also reduce the snap finger speed and rotational acceleration.
As a result, the researchers concluded that friction also has a "suitable range" — friction is too small to have enough energy to power snapping fingers; and too much friction causes energy to dissipate because it takes longer for the fingers to slide over each other, wasting the stored energy in the form of heat.
In fact, we are much better than Thanos
Not only that, but the scientists also took a closer look at the ordinary ringing fingers that humans make when they don't wear any gloves, and established a universal framework to explain the mechanisms behind them.
In the experiment, the researchers measured that the maximum rotational speed of the bare hand to beat the finger can reach 7800° per second, and the rotational acceleration is as high as 1600000° per second. Although this rotational speed value has not yet reached the measurement of the fastest rotational motion currently observed in humans, its instantaneous acceleration is the fastest human angular acceleration measured to date, almost three times the rotational acceleration of the professional baseball pitcher's arm.
"The first time I saw this data, I literally jumped out of my chair," Bhamla says, "and it only takes 7 milliseconds for a finger to snap, which is more than 20 times faster than blinking (it takes more than 150 milliseconds)."
In fact, as early as around 300 BC, ancient Greek art has depicted the movement of human fingers, and before that, humans may have been ringing fingers for hundreds of thousands of years.
Figure | (a) a piece of pottery from around 320 BC depicting the ringing of the finger; (b) a composite image of the movement at different points in time when the finger is tapped from the side; (c) measurement of the movement of the finger at three points in the wrist, knuckles, and fingertips; and (d) an illustration of the movement of the ringing finger.
The researchers believe that this result may provide multiple opportunities for future research, including understanding why humans are born to snap their fingers.
Only humans can snap their fingers?
The principle of snapping fingers is actually called cavity resonance in acoustics.
When the finger is snapped, the thumb and middle finger squeeze against each other, creating a large friction. To overcome this friction, the finger muscles need to provide a lot of force, and when the two fingers are staggered, the middle finger gains more momentum and hits the base of the thumb. At this point, the three parts of the middle finger, the ring finger, and the root of the thumb form a cavity. The middle finger quickly compresses the cavity, causing the air in the cavity to vibrate, producing a crisp "popping" sound.
So the question is, can only humans play the ring finger?
In fact, although only humans have been found to be able to snap their fingers, animals with hand structures similar to humans have theoretically hoped to learn to snap their fingers. Chimpanzees, macaques and other animals already do likes, "666" and other actions.
There is another animal that is not part of the primates, but they may also have the ability to snap their fingers – our national treasure, the giant panda.
The giant panda has a total of 6 fingers, and the "thumb" used to hold the bamboo has no knuckles, only a bone, growing on the outside of the real thumb, so it is called a pseudo-thumb.
This bone is actually a specialized wrist bone, but it can grasp things as flexibly and conveniently as a human or monkey. Therefore, giant pandas may also replace our human thumbs with "pseudo-thumbs" and make a beautiful ring finger.
However, you don't have to pay too much attention to which animals can snap their fingers, after all, it is more important to find out whether they will snap their fingers first
Reference Links:
https://royalsocietypublishing.org/doi/10.1098/rsif.2021.0672
https://www.eurekalert.org/news-releases/934621
https://www.youtube.com/watch?v=ycfLurEqmj4&ab_channel=GeorgiaTech