Slinky physics question
Discussion
speedy_thrills said:
Right, slinky on an escalator running at the same speed the slink falls. Slinky is made out of a perfect material and is so accurately aligned that it will never touch the side of the escalator. Does the slinky go on indefinitely down the escalator?
Yep, the rising escalator raises the potential energy of the slinky, the slinky turns this into kinetic energy by dropping to the next step.
In terms of the engery relationship between the slinky and the escaltor it remains in equilibrium, so long as the escalator continues to rise the slinky will continue to drop.
Should the escalator stop, the slinky could continue down all the steps of the escalator unti lit reaches ground level from where there is no other step from which it can drop.
TheExcession said:
Yep, the rising escalator raises the potential energy of the slinky, the slinky turns this into kinetic energy by dropping to the next step.
Surely friction will also cause a small amount of engery to be lost from the system as heat, over time that will dissapate the engery from the system and the slinky will eventually stop.
axeman30 said:
Yep, the rising escalator raises the potential energy of the slinky, the slinky turns this into kinetic energy by dropping to the next step.
Surely friction will also cause a small amount of engery to be lost from the system as heat, over time that will dissapate the engery from the system and the slinky will eventually stop.
Hmmm, you might be right..... I guess the escalator might have to be running a tiny bit faster in order to add that tiny extra amount of energy, but if we assume a lossess/perfect system (as one is often want to do in these circumstances) then it would continue for as long as the escalator was moving.
ETA:
In fact we must consider the energy input required to get the spring to move off the first step.... this (the input energy) is the energy that as you say would eventually be lost through friction etc.
The speed of the escalator could be increased to ensure that this loss was compensated for.
>> Edited by TheExcession on Thursday 8th December 11:51
axeman30 said:
Yep, the rising escalator raises the potential energy of the slinky, the slinky turns this into kinetic energy by dropping to the next step.
Surely friction will also cause a small amount of engery to be lost from the system as heat, over time that will dissapate the engery from the system and the slinky will eventually stop.
That's only going to happen if you are thinking of the elevator as 'coasting' with nothing to drive it. I think even in this theoretical experiment you'd assume something was driving the escalator at constant speed, and this would supply the energy the slinky is losing as well as the energy lost in friction etc within the escalator.
The key question is where the energy for the flipping motion of the slinky comes from. If it is a product of the initial 'flick' to get it started then this will decay over time due to losses and the slinky will eventually stop. However, if it is a product of the conversion of potential energy to kinetic energy then it should continue.
I think the main reason that many people are thinking "no" as a gut reaction is that they're thinking of perpetual motion. This isn't the case, though, as the escalator is supplying energy and is analagous to an infinitely long stairway.
>> Edited by JonRB on Thursday 8th December 12:02
I think the main reason that many people are thinking "no" as a gut reaction is that they're thinking of perpetual motion. This isn't the case, though, as the escalator is supplying energy and is analagous to an infinitely long stairway.
>> Edited by JonRB on Thursday 8th December 12:02
JonRB said:That’s what I’m thinking but somehow I’m having trouble visualising the vertical momentum being turned into horizontal momentum, I think it’s probably being acted on by an internal momentary force to keep the conversion of horizontal potential energy to lateral motion. This conversion is certainly possible as a marble can role down a hill.
The key question is where the energy for the flipping motion of the slinky comes from. If it is a product of the initial 'flick' to get it started then this will decay over time due to losses and the slinky will eventually stop. However, if it is a product of the conversion of potential energy to kinetic energy then it should continue.
Naturally if the system was regarded as perfect and without losses you would not need steps, the initial shove would supply the energy needed so it could slink along a flat floor.
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