Why can martial artists make steel needles penetrate glass? Some people who have practiced martial arts possess a unique skill—they can throw steel needles with their bare hands and pierce glass, while ordinary people struggle to even leave a trace on the glass even with all their strength. So does "steel needles penetrating glass" require some special trickery? What are the key factors involved? Using professional speed measurement instruments or high-speed cameras, it can be determined that the speed required for a steel needle to penetrate glass is approximately 30 meters per second, or 100 kilometers per hour. Is this speed really that high? Surprisingly, most people don't realize that most adults can throw steel needles at or exceed this speed! This shows that the most critical factor in "steel needles penetrating glass" is not speed. In fact, to penetrate glass, the steel needle must hit the glass vertically, so that its momentum can be fully converted into the impulse needed to pierce the glass. During performances, martial arts experts swing their arms forcefully, causing the steel needles to tumble as they fly. Therefore, getting the steel needle to hit the glass vertically is extremely difficult! Only through years of practice can one master the timing of the throw and the strength of the wrist flick with great skill. Ordinary people lack this experience and naturally cannot control the angle at which the steel needle hits the glass.
Some might still doubt the importance of angle. Let's look at the following example: Soldiers' helmets are almost spherical, so if a bullet doesn't hit the helmet vertically, it will bounce off. This is because the momentum is dispersed during an oblique impact. Some might wonder: Since angle is so important but hard to control, maybe I could just throw dozens of times and hopefully hit the glass at a vertical angle by chance? The answer is largely negative, as the probability is too low and our arms would tire out after dozens of attempts. Of course, there's a little trick: sharpening the tip of the steel needle can help it penetrate glass. If the tip is too sharp, it may create too much pressure during impact, but the deformation of the needle tip will absorb energy and make it harder to penetrate the glass.
There may be parallels between the story of Li Guang shooting stones and the ability to make steel needles penetrate glass. According to the "Biography of General Li Guang" in the "Records of the Grand Historian": "When Guang went hunting, he saw a stone in the grass and thought it was a tiger, so he shot at it. The arrowhead penetrated the stone and was embedded in it. Upon closer inspection, he realized it was just a stone. He tried again, but failed to penetrate the stone." Li Guang thought he encountered a tiger and, in his panic, pulled the bowstring with great force, eventually embedding the iron arrowhead into the stone. If we refer to the factors involved in making steel needles penetrate glass—arrow speed, arrowhead sharpness, and shooting angle—it's clear that the irregular surface of the stone makes controlling the shooting angle even more challenging. Li Guang's initial success was likely a fluke, which is why he couldn't repeat it later.
In summary, people often misunderstand "steel needles penetrating glass" and focus solely on increasing arrow speed while neglecting angle control, which is why they fail repeatedly. However, martial arts experts, through years of training, can simultaneously ensure arrow speed and control the angle of the steel needle, which is the key to making the needle penetrate glass.