wheelspinner
Are We There Yet? Member
Nobody's perfect, I'm a nobody, so ...
Posts: 4,103
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Post by wheelspinner on Apr 19, 2012 5:33:14 GMT -5
We've all played with a slinky, right? Here's an experiment for you.
Hold a slinky up, dangling it straight down from your hand. Let go of the slinky. How does it fall?
Will it:
1/ fall straight to the ground
2/ have the bottom of the slinky spring up to the top, and then fall
3/ have the top of the slinky come down to the bottom, and then fall
4/ have the top and the bottom both retract, meet in the middle, and then fall.
I got got it wrong. Do you know the answer?
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Post by Peltigera on Apr 20, 2012 13:06:07 GMT -5
Impossible to tell from the info given.
The slinky will contract. In the absence of gravity, both ends would move towards the middle. Correct answer depends on the actual tension in the spring, which is not given. Because slinkies are not very strong springs, I suspect the motion of the bottom moving towards the middle will be a lot less than the motion of the bottom falling under gravity.
My complete answer: the slinky will fall straight to the ground, with the top moving down faster than the bottom.
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Post by Peltigera on Apr 21, 2012 7:51:37 GMT -5
A change to my answer above. If the slinky is stretched due to its own weight, then the bottom will not move towards the middle. So the slinky will fall straight to the ground but still with the top moving faster than the bottom due to the slight tension in the slinky.
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wheelspinner
Are We There Yet? Member
Nobody's perfect, I'm a nobody, so ...
Posts: 4,103
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Post by wheelspinner on Apr 21, 2012 18:18:58 GMT -5
Your last comment is correct. The bottom of the slinky stays where it is while the top comes down to meet it. When the top meets the bottom the slinky then falls to the ground.
The best way I can explain it - and I'm no physicist - is that the potential energy stored in the spring is converted into kinetic energy under the force of gravity. So the spring has to fully contract before then falling completely.
The tension in the spring actually makes no difference to the result. I've seen this done with just the slinky dangling, and with a tennis ball attached to increase the tension. Same result.
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