Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: dropped slinky



A simplified first model:
Consider two equal masses joined by a spring of negligible weight. Let
the system hang from your hand. The upper mass is in equilibrium under
the influences of your upward force (F), the downward pull of the upper
weight (mg), and the downward pull of the stretched spring (kx). The
lower mass is in equilibrium under the downward pull of its weight (mg)
and the upward spring force (kx) - so that kx = mg. If your supporting
force suddenly vanishes, the acceleration of the upward mass is downward
and equals (kx + mg)/m = 2mg/m =2g.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "Carl E. Mungan" <mungan@USNA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, November 22, 2002 1:29 PM
Subject: dropped slinky


| Gene Mosca mentioned the following interesting tidbit to me.
|
| Hold a slinky from the top so that it stretches down vertically
| (while standing on top of a table so the bottom end is free). It's
| pretty well known that if you now release it, the initial
| acceleration of the bottom end is zero. (The slinky collapses from
| the top down before the bottom end begins to fall substantially. Try
| this yourself if you haven't seen it before.)
|
| The interesting new question is: What is the initial acceleration of
| the top turn? Suppose the slinky has 100 turns.
|
| The answer, which is as surprising as the zero for the bottom end, is
| about 100g! (The top turn has 1/100 of the mass but has to support
| the weight of the entire mass initially. Or consider a mass-weighted
| average acceleration where only the top turn initially is out of
| equilibrium.)
|
| One might consider this a "slowed down" model for compressional waves
| traveling through a solid rod when something sudden happens at one
| end (hits the ground, dropped from a suspension point, etc).
| --
| Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
| U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
| mungan@usna.edu http://physics.usna.edu/physics/faculty/mungan/
|
| This posting is the position of the writer, not that of SUNY-BSC, NAU or
the AAPT.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.