Re: test problem

[Bob Sciamanda <trebor@VELOCITY.NET>]

I don't understand what any of you are doing!
As I read the problem (there is some ambiguity) , the mass will oscillate
with an amplitude of  20 cm about the equilibrium position (the position
where the net force on it is zero).

(I read the problem as stating that the mass is released from a point 20
cm below this equilibrium position - ie: from the point 25 cm below the
unstretched spring position.)  !?!?

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "Rick Tarara" <rtarara@SAINTMARYS.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, December 04, 2001 3:37 PM
Subject: Re: test problem


> It seems that both you and John M have taken the displacement of the
spring
> from the totally unstretched position.  I'm not sure that is correct.
The
> equilibrium position is with the spring stretched 5 cm and therefore at
the
> extreme, the spring is stretched 20 cm from the equilibrium.  Isn't that
> what should go in the 1/2kx^2 term?   That is, the energy equation would
be:
> mg(-.25m) + 1/2k(.20m)^2 = mg(-.125m) + 1/2k(.075m)^2 +1/2mv^2
>
> This yields a lower value for v than your 2.6 m/s.  This seems related
to
> problem 10-10 in Hecht's Calculus text (which I had my class do and is
the
> only reason I question this--being immersed in papers right now).
>
> Rick
>
> **********************************************
> Richard W. Tarara
> Professor of Physics
> Saint Mary's College
> Notre Dame, IN 46556
> rtarara@saintmarys.edu
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> ----- Original Message -----
> From: "Justin Parke" <FIZIX29@AOL.COM>
> To: <PHYS-L@lists.nau.edu>
> Sent: Tuesday, December 04, 2001 2:36 PM
> Subject: test problem
>
>
> > I recently did a conservation of energy lab with my high school gifted
and
> talented physics students.  The lab involved a mass oscillating on a
spring,
> locating the top and bottom displacements for a vertical oscillation,
and
> comparing the potential energies (elastic and gravitational) at these
> points.
> > Part of the analysis was to compute the potential energies for the
> midpoint of the oscillation and find the difference between this value
and
> the value at the top of the oscillation, and then to use energy
conservation
> to find the speed of the mass as  it passed through the midpoint.  Only
a
> few diligent students have asked me any questions about this and I
suspect
> the rest just made up some numbers.  These diligent students found that
the
> total potential energy was greater at the midpoint than at the top,
which
> would lead to negative kinetic energy.  I trust that they took
reasonably
> accurate measurements.  Did I miss something in my writing of the lab?
> > Along the same line, I wrote the following question for a test on
energy
> and momentum, related to the lab.  I get 2.6 m/s.  In your (plural)
opinion,
> is this a reasonable test question?
> > Thanks as always for your (plural) input.
> >
> >
> > Test question:
> >
> > "This question is related to the conservation of energy lab:  a 1.0 kg
> mass is hung from a spring which is then observed to stretch 5 cm.  The
mass
> is then pulled down 25 cm from the unstretched length of the spring and
> released from rest.
> >     a)  find the spring constant of the spring
> >     b)  find the total potential energy (spring and gravitational) of
the
> system after it is stretched down.  (Define the origin to be the bottom
of
> the unstretched spring.)
> >     c)  Use conservation of energy to find the speed of the mass as it
> passes through the midpoint (i.e. 12.5 cm from the origin)"