Re: Help!! etc.

["Dewey Dykstra, Jr." <dykstrad@varney.idbsu.edu>]

> Isn't the spike on the acceleration graph described nothing more than
> the "jerk" that results as the cart goes from a condition of zero net
> force to one where the falling weight provides the net force?  I have
> had excellent success with the Pasco carts and the motion detectors,
> even the "noisy" acceleration graphs make sense to students as they
> continue to examine these "imperfect" conditions.
> Pat Callahan
> Catasauqua HS, PA

Actually why should the acceleration do anything but change from zero to
some constant value if the net force changes from zero to some constant
value?  What we see in the graphs is that the acceleration appears to
over-shoot and settle back down to some more or less constant value.

Does "jerk" cause acceleration?  I thought "jerk" was a kinematic entity
(3rd derivative of displacement with respect to time) and not a dynamic
one.  It strikes me that "jerk" is the change in acceleration associated
with a change in the net force.

I can easily imagine a small vibration in the pulley scaffolding happening
when the cart is released.  The force on the probe drops slightly when the
tension drops as the cart is released.  One might expect a small drop in
acceleration as the pulley scaffolding responds to this.  It might be that
the proportionality between the force and the acceleration is such that one
sees almost no disturbance in the tension as indicated by the force probe,
but can still see it in the acceleration graph.  BUT one would expect the
acceleration to 'settle down' into its original value after this slight
drop.  INSTEAD what is often seen is an initial peak in the acceleration
and then a settling down into a LOWER acceleration.  What is actually seen
here is more suggestive of a start-up problem and does actually appear to
be minimized when more attention is paid to start-up technique.

Dewey


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Dewey I. Dykstra, Jr.                   Phone: (208)385-3105
Professor of Physics                    Dept:  (208)385-3775
Department of Physics/SN318             Fax: (208)385-4330
Boise State University                  dykstrad@varney.idbsu.edu
1910 University Drive                   Boise Highlanders
Boise, ID 83725-1570                       novice piper
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