How a slinky falls in slow motion #shorts
Why Physics Stack Exchange recommends it
Physics Stack Exchange answerers point people to it as a visible demonstration that forces and information move through materials at the speed of sound, not instantly, when explaining rigid-body, pulley and bridge thought experiments.
Recommended by 3 people across 3 different months, 2021 to 2025.
What it is
A 42-second Veritasium short showing a slinky released from the top in slow motion. The bottom of the slinky hangs in place until the collapse wave reaches it, which shows that disturbances in a solid travel at a finite speed. The description extends the point to steel rods, tennis rackets and golf balls.
What people said
Recommends Alwin · 2021-07 · Physics Stack Exchange · on “Speed of light through experiment”
See this 40 second video: [this video] The bottom of the slinky doesn't even know the top of the slinky has been let go. There is a delay because any given atom in the material only cares about its distances to its neighboring atoms, which determines the forces that atom feels. Most forces are based on distances. Imagine a rope ABCD Letting go of the rope at A will change the forces on just atoms of A. After those atoms move, the forces between A and B will change, so only then will B move. Only then will the forces between B and C change, so only then will C move. You can think of this as a domino effect. This is a basic explanation of how materials have a speed of sound, the speed at which this
Recommends RC_23 · 2024-12 · Physics Stack Exchange · on “Will Pulley Rotate Instantly, even after being separated by a distance more than a light second?”
Pressure waves in material objects, including transmitted forces, travel at the speed of sound in the object, which is based upon the density and rigidity of the material. A material with one of the highest speeds of sound is diamond, at about 12,000 m/s, while most other solids are a few thousand m/s. So any forces in material objects will travel at most this fast, which is far slower than the speed of light. The far end of your pulley system will not "know" anything about forces imparted to the far end until the pressure waves traverse the distance through the rope. A good demonstration of this is with a slinky [this video] , which due to how loose and "unrigid" it is, has an effective speed of
Recommends J. Delaney · 2025-01 · Physics Stack Exchange · on ““Reality” of length contraction in SR vs. Bell's spaceship paradox”
Consider the midpoint of the thread: when the spaceships start accelerating, it will not feel any force for a time of at least $L/c$ , since nothing can propagate faster than light ( $L$ being the distance from the midpoint to the spaceships). That means that this point will remain at rest for some time while the endpoints are moving, so the thread will be stretched on one side and compressed on the other. In fact this is not even a relativistic effect, because in reality the pulling force propagate through rigid objects at the speed of sound, much slower than the speed of light: if you pull a metal rod for example faster than the speed of sound in that metal, it will also stretch. (not exac
Cites RC_23 · 2023-08 · Physics Stack Exchange · on “Relativity on a massive bridge”
You may be interested in this experiment [this video] . The bridge, however long (it doesn't have to be light seconds) will collapse in the same way. This is not even a relativistic scenario, as the governing speed is the speed of sound, ~3000 m/s for most solids. The speed of sound in solid governs how fast changes in intermolecular forces propagate. The effective speed of sound in the slinky is very slow, allowing us to see it happen. As the narrator notes, different wave modes (such as twisting, shear, compression) can have different sound speeds.