I have a physics degree and that's not really what I would have expected to happen! But we may not be able to see clearly that the bottom is still falling a little bit – and the center of mass is still accelerating towards the Earth at 1g it's masked by the collapsing of the spring which is part of that COM movement.
Nah, the bottom is not falling. Force propagates through a physical object at a speed specific to the material. The bottom doesn't know the top has been dropped before it catches up. Until then, the top is still exerting the same pull on it it did when it was hanging.
It's actually the same reason you wouldn't be able to use a very long pole to communicate faster than light (like, it's a one-lightyear long pole, suspended in zero g, and you push and pull on one end to transmit information – it doesn't work because the push or pull is actually a sound wave travelling down the pole, and it will travel at the speed of sound in that medium (material).
I think we should visualize it as a slinky that is shrinking in length on release and also falling when we consider its center of gravity. For the bottom, the rate of moving up due to shrinkage, and rate of falling are same, and hence it appears to be stationary.
+Sowmyan Tirumurti indeed. If you look at the frame of reference of the center of mass – the middle of the spring – the bottom is moving up but as you say, only as fast as the COM moves down.
There is some non linear expansion of the slinky. The top parts are weighed down more than the bottom part. It is not exactly like a spring with equal expansion of all segments, when the load is far greater than the weight of the spring. For each section the weight of the slinky below that section is the load and this changes along the length.
Nah, this is not about speeds coincidentally aligning.
Again, it is about the speed of propagation. Imagine if the slinky was one light year long. How fast will the information that the top has been released reach the bottom? It is not instantaneous, and until it has propagated, nothing about the bottom's behavior changes.
Again, if it weren't like this, it could be used for FTL communication.
I have a physics degree and that's not really what I would have expected to happen! But we may not be able to see clearly that the bottom is still falling a little bit – and the center of mass is still accelerating towards the Earth at 1g it's masked by the collapsing of the spring which is part of that COM movement.
Nah, the bottom is not falling. Force propagates through a physical object at a speed specific to the material. The bottom doesn't know the top has been dropped before it catches up. Until then, the top is still exerting the same pull on it it did when it was hanging.
It's actually the same reason you wouldn't be able to use a very long pole to communicate faster than light (like, it's a one-lightyear long pole, suspended in zero g, and you push and pull on one end to transmit information – it doesn't work because the push or pull is actually a sound wave travelling down the pole, and it will travel at the speed of sound in that medium (material).
Good 1000fps illustration of the center of mass movement –
https://www.youtube.com/watch?v=59dANJTLbyo and a supersize version here https://www.youtube.com/watch?v=JsytnJ_pSf8
I think we should visualize it as a slinky that is shrinking in length on release and also falling when we consider its center of gravity. For the bottom, the rate of moving up due to shrinkage, and rate of falling are same, and hence it appears to be stationary.
+Sowmyan Tirumurti indeed. If you look at the frame of reference of the center of mass – the middle of the spring – the bottom is moving up but as you say, only as fast as the COM moves down.
There is some non linear expansion of the slinky. The top parts are weighed down more than the bottom part. It is not exactly like a spring with equal expansion of all segments, when the load is far greater than the weight of the spring. For each section the weight of the slinky below that section is the load and this changes along the length.
Nah, this is not about speeds coincidentally aligning.
Again, it is about the speed of propagation.
Imagine if the slinky was one light year long.
How fast will the information that the top has been released reach the bottom?
It is not instantaneous, and until it has propagated, nothing about the bottom's behavior changes.
Again, if it weren't like this, it could be used for FTL communication.
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