1. Bullets vs. Glass Cracks: Which Is Faster?
The video creators The Slow Mo Guys measured the speed of the tip of a crack in thin glass and a bullet using the same high-speed camera. The first filming session used 80,000 frames per second. The results showed the crack tip at about 1,436 m/s (about 3,214 mph) and the bullet at about 343 m/s (768 mph).[4] Dividing 1,436 by 343 gives about 4.2, so the crack was more than four times faster than the bullet.
However, this is a single measurement from a video for general audiences. Values change depending on the type and thickness of the glass, so it is safe to interpret this as "glass cracks can reach speeds in the range of 1,000 m/s."
2. How Did We Film a Crack Moving in 0.0007 Seconds?
Since the crack tip moves at about 1,436 m/s, it takes only about 0.0007 seconds to travel from one end of a 1-meter window pane to the other. At this speed, human eyes only see the moment it breaks.
High-speed cameras made this movement visible. Filming at 80,000 frames per second means the interval between one frame and the next is 1/80,000th of a second. In that time, the crack moves about 1.8 cm (1436 ÷ 80,000). Across a window pane, this can be seen split into dozens of frames.
There were also methods to capture invisible moments. Harold Edgerton (1903–1990), a professor of electrical engineering at the Massachusetts Institute of Technology (MIT) in the US, photographed the moment milk drops hit a plate and bullets pierce apples using strong strobe lights.[4] The MIT Edgerton Center states that strobe light can be shortened to units of one-millionth of a second. If the light lasted for 1/1,000,000th of a second, the crack would move about 1.4 mm (1436 × 0.000001).
3. Why Does a Crack Have a Speed Limit and Branch When Too Fast?
The speed of a crack has a "theoretical limit" determined by the speed of waves traveling through the material. For glass and acrylic resin (PMMA), the actual maximum crack speed is said to be about half that limit, with values ranging from 0.5 to 0.7 times depending on the source.[4]
Branching of the crack tip (microbranching) is thought to appear when the speed reaches about 40% of this limit. This means that as cracks get faster, they may branch out. Cracks become less like a single simple line as they accelerate.
What happens if you store force inside glass before it breaks? Such glass exists.
4. Prince Rupert's Drops: Why Is the Head Strong but the Tail Fragile?
Prince Rupert's drops (also called Dutch tears) are drops of molten glass cooled suddenly in cold water. The outside hardens and shrinks first, while the inside hardens later and pulls it, leaving the outer side strongly compressed and the inner side stretched.[4]
The round head is strong enough not to break even when hit, but the thin tail is a weakness. If the tail is slightly scratched, a crack runs from the tail to the head, and the whole drop shatters explosively.[4]
This collapse speed was measured at about 1,450–1,900 m/s in a study in 1994.[4] The American Ceramic Society describes it as up to 4,200 mph (about 1,880 m/s).[4] Because values vary by source, we do not settle on one number. Both are in the "range of 1,000 m/s," similar to the thin glass crack seen earlier, and about five times faster than a bullet (about 343 m/s).
This idea of "stored force" is also used in everyday glass.
5. How Does Tempered Glass Use Internal Forces to Resist Breaking?
Tempered glass is made by treating sheet glass with heat or chemicals to create a compressed layer on the surface.[4] Glass surfaces have tiny invisible scratches, and cracks start from there. If the surface is compressed, this spread is suppressed, making it harder to break than ordinary glass.
When it does break, it forms small fragments like beans. This is thought to happen because the surface is compressed while the inside is stretched, causing many cracks to form at once.
A 1991 paper by the Materials Research Society of Japan investigated where tempered glass fragments become fine. They analyzed this by "assuming" a crack tip speed of 1,500 m/s and an elastic wave speed from the plate edge of 3,300 m/s. The place where fragment size changes suddenly matched the position where the crack tip and waves from the edge collided.[4] (These two speeds were assumptions, not measurements.) The ratio of speeds in this analysis was about 0.45 (1500 ÷ 3300).
In 2021, AGC and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) announced a method to reproduce how chemically strengthened glass breaks using numerical calculations. Previously, trial and error was needed to optimize strengthening forces.[4]
Methods have been published not only to capture invisible moments with filming but also to reproduce breakage patterns with numerical calculations.
6. How Can You Feel the Speed with Smartphone Slow-Motion?
Try testing with water drops. Put water in a cup and film drops from a dropper or tap hitting the surface using your smartphone's slow-motion function. You will see the same kind of "moment" as Edgerton photographed with milk drops.[4] Lay cloth or newspaper on the table to avoid spilling water.
To feel crack speed by calculation: If your smartphone's slow-motion is set to 240 frames per second (varies by model), the interval between frames is about 1/240th of a second. In that time, a crack moving at about 1,436 m/s travels about 6 meters (1436 ÷ 240). A crack running through a window pane fits entirely within one frame. This shows that filming cracks requires different tools, like those used for 80,000 frames per second.
Do not try breaking glass yourself. Broken glass is dangerous and cleanup requires care.
If you want to read more: Film familiar "moments" with your smartphone and compare them. Besides water drops, watching a balloon deflate or a ball bounce frame by frame reveals shape changes. Calculating "how many cm moved per frame" from the frame count lets you estimate speed.