Re: test problem

[Brian Whatcott <inet@INTELLISYS.NET>]

At 01:36 PM 12/4/01, Justin wrote:
> ...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?
...
> 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)"


    Nit Picker's Special

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.

You're welcome

Brian W


Brian Whatcott
  Altus OK                      Eureka!