Re: Questions for those who really know their physics . . .
- From: Ben Bradley <ben_nospam_bradley@xxxxxxxxxxxxxxx>
- Date: Mon, 26 Dec 2005 04:39:16 GMT
On Sun, 25 Dec 2005 18:29:31 -0600, Wildepad <noreplies@> wrote:
>On Sun, 25 Dec 2005 09:39:10 -0700, Mark Fergerson <nunya@xxxxxxxx>
>wrote:
>
>>Wildepad wrote:
>>> 1) Will a Slinky work on an escalator?
If you can get a slinky to 'walk' down a long run of stairs, I have
no doubt it will also 'walk' down an operating escalator.
When I was substantially younger I had little success making a
slinky go more than one step or so (unlike the commercials for it I
saw at the time, though I don't recall how many consecutive steps they
showed it walking down), perhaps due to the step size, but that
experience did put doubt in my mind that a slinky can walk down steps
indefinitely.
>>> 2) If so, how much is the 'tumbling' speed be affected by temperature
>>> (i.e. if I build a desktop escalator with a miniature slinky on it,
>>> could it be preset or would there have to be sensors to detect if the
>>> Slinky is staying near the middle and adjust the speed accordingly)?
>>
>> You might need to find the temperature dependence of the modulus of
>>elasticity of whatever your slinky is made of (plastic [yucch!] vs.
>>steel), but I don't think it'd be that big a deal unless your desk is in
>>an environment subject to drastic temperature shifts.
>
>(I share your opinion of the plastic ones.)
>
>The problem as I see it is that even a tiny change will cause problems
>because of the large number of increments.
>
>If there is even the slightest difference between its period at 74
>degrees (standard for offices in summer) and 68 degrees (winter
>standard), then that 'error' will accumulate significantly in light of
>the fact that the steps will only take about two seconds each, but the
>machine will probably be left running 24 hours a day, seven days a
>week.
If you think the slinky's step period can be measured accurately
enough at the two temperatures (and at a few in between to see the
shape of the temperature/period curve), then 'theoretically' you only
need to change the escalator speed with temperature.
But I don't see this as practical. An analog tape recorder can't
play back a five-minute tape twice without a variation of a half
second or more in timing, even with the best servo mechanisms driving
the capstan motor. The slinkly will likewise drift up or down the
escalator, due to various reasons, perhaps humidity, the 'wear' of the
slinky (steel pieces will rub against each other, removing microscopic
amounts that will add up over time and affect period), and probably
other parameters I can't think of.
The escalator speed will have to be servo'ed to the slinky position
for this thing to run more than perhaps a few minutes at a time.
>
>> This has also already been touted as an executive-toy "Newton's
>>Cradle for the 21st century".
>
>What I could find on that is just a joke, not a serious intention to
>build one, and it appeared years after I first postulated it. I'm
>almost offended that they did not also copy my variations.
.
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