Does a Falling Slinky Defy Gravity?
Why Physics Stack Exchange recommends it
Physics Stack Exchange answerers link it as a vivid demonstration that the centre of mass falls at g while the bottom of a spring does not move until information from the top reaches it.
Recommended by 4 people across 4 different months, 2015 to 2021.
What it is
A short Veritasium video that shows, in slow motion, a hanging slinky being released and collapsing. It explains that the centre of mass accelerates at g while the bottom stays put until the compression wave from the top arrives, and it points to an accompanying physics paper.
What people said
Recommends Skyler · 2015-05 · Physics Stack Exchange · on “Is there any tension in a massless spring that connects two free falling bodies in different horizontal planes?”
The answer is that it depends on how your initial spring loaded mass is moving. But , the fascinating (but not too fascinating once you phrase it like this) part is that until the compression wave from the top interacts with bottom out on the slinky the dynamics of the bottom half won't change. If we assume it was at rest, essentially the top mass will move fast enough that the center of mass will accelerate at $9.8 m/s^2$. When it reaches the true equilibrium length of the string without gravity it will start accelerating the bottom mass. At this point if you looked from the COM frame the spring would appear to be oscillating like it normally does. This is because the oscillations occurring
Recommends David White · 2016-02 · Physics Stack Exchange · on “Does deformation due to a force affect the acceleration produced by it?”
@Asher , fair enough. Watch the falling slinky [this video] . The center of mass still accelerates downward at g, even though many parts of the slinky do not accelerate at all.
Recommends Clara Díaz Sánchez · 2019-12 · Physics Stack Exchange · on “Conversion of elastic potential energy in free fall”
The center of mass of the spring+mass system will accelerate downwards at $g$ . During this motion the spring will contract, so the top end of the spring will accelerate faster than $g$ , and initially the bottom end will hardly accelerate at all because at the initial moment the spring tension and the weight exactly cancel. There are some nice videos on youtube which show this effect ( [this video] for example) - the lower end of the spring seems to "defy gravity" for a while. So as usual the potential energy of the spring is being converted into kinetic energy. When the spring is fully contracted it will expand again, performing simple harmonic motion in the rest frame of the center of mass. It
Recommends BioPhysicist · 2020-08 · Physics Stack Exchange · on “Is the effect of friction force transferred INSTANTLY in this case?”
You might like this video [this video] . But to get to an actual comment here, your scenarios are somewhat ill-defined and inconsistent. For example, you talk about a rope with Bob, then then talk about a ball. Then you switch from losing friction to suddenly changing the length of ropes. Can you please clean up your question to focus on a single, well-defined scenario?
Cites BioPhysicist · 2019-08 · Physics Stack Exchange · on “Why does nature prefer simultaneous events?”
None of the processes you describe are instantaneous. Acceleration is produced without any delay on applying force. Angular acceleration is produced without any delay on applying torque. If you are looking at the microscopic scale, it takes time for fields to change in order for forces to be produces. For example, E&M changes propagate at the speed of light. If you are looking at macroscopic bodies, there is no such thing as a rigid body. "Information" about the presence of a force propagates through the body at the speed of sound in the body. This propagation also takes time. A cool example of this is shown in this video [this video] bulb glows simultaneously as we close the circuit No, it actua
Cites user288901 · 2021-03 · Physics Stack Exchange · on “Gravitation in solar system”
It takes about 8 minutes for light from the sun to reach us. Gravity is a field whose disturbances travel at the same speed as light. 8 minutes after the sun is gone the Earth would be released from its orbit and keep going until it finds itself in some other orbit or not. This is similar to how the bottom of a hanging Slinky does not instantly know that the top has been released. That information has to travel to the bottom of the slinky before it can fall [this video] .