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Re: test problem



Actually with the 5 spring set I use in lab, we have to load the springs
with .1-.5 kg in order to unbind the coils (tested by seeing if the hanging
mass will 'bounce' just a bit or by squinting to see if any light can be
seen through the coils). In a Hooke's law/SHM lab, we then consider the
stretching of the spring ONLY from this initial 'unbound' position. That is
the initial force and the initial displacement are taken as zero even
though some mass has been added and the spring has stretched. Masses are
added, displacements measured, and EXTREMELY linear plots are produced and
used to find the spring constants. Then a kilogram or more is hung from the
springs, put into SHM, periods measured and compared to T = 2pi(m/k)^.5
with m = m-hanging + 1/2 m-spring. It is the one lab of the semester where
there is really good agreement between experiment and theory--a pleasant
change! ;-)

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

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----- Original Message -----
From: "Brian Whatcott" <inet@INTELLISYS.NET>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, December 05, 2001 12:58 PM
Subject: Re: test problem


At 05:41 AM 12/5/01, you wrote:


In a message dated 12/4/2001 11:17:01 PM Eastern Standard Time,
inet@INTELLISYS.NET writes:


No, the questions are unreasonable. If the extra downward displacement
is
greater than the equilibrium displacement, then the possibility of
coil
binding is not excluded on the upward rebound.
For this same reason, the spring constant is not determined by
only the
force required for a specified displacement.
To summarize: a) and b) are indeterminate, and c) is incorrectly
modeled.


Does my specification of an origin (and by implication a reference for
zero
gravitational potential energy) remove the indeterminacy of b)?

Justin


As you have seen from other replies, if you assume a loose coiled spring,
the initial force to extend it starts at zero. Here, the spring rate may
be
found. The pathological case about which I also nitpicked, is the
close coiled ("coil-bound") spring, for which the force to
initally extend it is non zero. These springs are not uncommon
(except among physics teachers, it seems)

Brian

Brian Whatcott
Altus OK Eureka!