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# Mechanics is easy Tweet

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1. Mechanics is easy
A long slinky is hanging from a piece of string tied to the ceiling. The end of the slinky is a short distance above the floor. The string at the top is cut. Will the initial movement of the bottom of the slinky be

a) upwards
b) downwards
c) doesnt move
d)I need relativistic quantum mechanics to solve this.
2. Re: Mechanics is easy
I'm making a guess at c) doesn't move, althought that is just an educated guess, if not d)
3. Re: Mechanics is easy
(Original post by teachercol)
A long slinky is hanging from a piece of string tied to the ceiling. The end of the slinky is a short distance above the floor. The string at the top is cut. Will the initial movement of the bottom of the slinky be

a) upwards
b) downwards
c) doesnt move
d)I need relativistic quantum mechanics to solve this.
I believe it to be a)
Given that the mass of the slinky is appreaciably large compaired to the spring constant.
Consider. The top of the slinky, intially is in equalibrium.
It has a force below it of integral of rho g dl, downwards.
It has therefore a force above it of integral rho dl.
We remove the supporting force. The slinky still has in it stored the force rho g dl pulling on the top of the slinky downwards. Hence its springs downwards.

The top comes down, apply Newton's 3rd and Symmetry, the bottom must come up.
4. Re: Mechanics is easy
This is a situation that's easy to check by experiment.

9 out of 10 Physics lecturers got it wrong when asked (me included)
5. Re: Mechanics is easy
ive changed my mind, i think its moves downards!!!!!!! haha !
6. Re: Mechanics is easy
(Original post by teachercol)
This is a situation that's easy to check by experiment.

9 out of 10 Physics lecturers got it wrong when asked (me included)
Experiment? What are they?
7. Re: Mechanics is easy
presumably an experiment where u attach a slinky to the roof and then cut it and LOOK!
8. Re: Mechanics is easy
Yeah - I had to video it and replay in slow motion to convince people what happened.
9. Re: Mechanics is easy
so what was the result of the experiment?
10. Re: Mechanics is easy
The bottom of the spring remains stationary. It looks incredible like its floating in the air.

The top falls towards the bottom. When the spring is no longer stretched , it drops to the floor.

Think of it like this. When you cut the string, the bottom doesnt 'know' about it yet. The only change is that there is nothing supporting the top loop, so it falls. Then the second to top loop falls and so on. The info propogates through the spring at the speed of 'sound' .

It makes no difference if you hang a weight on the bottom and thats even stranger to watch.
11. Re: Mechanics is easy
wehey i was right first time!! I'll pretend i didn't change my answer! lol!
12. Re: Mechanics is easy
So it didnt move? I wouldve thought it didnt move, but mehh made me think otherwise....
13. Re: Mechanics is easy
(Original post by r0manticide17)
So it didnt move? I wouldve thought it didnt move, but mehh made me think otherwise....
I did preceed my thoughts with the disclaimer "I think..."
14. Re: Mechanics is easy
I think this question was an oxbridge question. I think it's to do with the fact that information has to take time to transmit to the bottom?
15. Re: Mechanics is easy
(Original post by darkenergy)
I think this question was an oxbridge question. I think it's to do with the fact that information has to take time to transmit to the bottom?
So for a rigid material. Say a steel rod hanging vertically from a string. Would this happen on a quantum level then? And hence very fast? Atoms in a crystal structure do have a (very confined) elasticity / degree of freedom. What speed would the information travel in a ceramic? Would it travel as phonon/acoustic energy?
16. Re: Mechanics is easy
(Original post by ApeXaviour)
So for a rigid material. Say a steel rod hanging vertically from a string. Would this happen on a quantum level then? And hence very fast? Atoms in a crystal structure do have a (very confined) elasticity / degree of freedom. What speed would the information travel in a ceramic? Would it travel as phonon/acoustic energy?
no idea! I am no physicist (and proud of it )
17. Re: Mechanics is easy
For a steel rod distance the information has to move through is much shorter, thus it will reach rod end very fast. Additionally, sound speed in metal rod is much higher (at least its vertical component we are interested in - no idea what the slinky is made off).
Last edited by GuerillaBoy; 28-03-2006 at 09:45.
18. Re: Mechanics is easy
Is anyone willing to explain the maths behind this?