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Re: Help!!An interesting acceleration graph



Dewey,

did you really mean to say,

The 'scaffolding' experiences 'wind up' at the catch so
there is quite a vibration in the force graph at the end, but not in the
acceleration graph.

I may have missed something and therefore be off-base with this comment,
since I haven't directly dealt with this apparatus, but if this a graph of
force (net force?) and a graph of its acceleration, the two should be the
same (proportional to each other, actually) and vibrations problems for the
two the same. Did you really mean a velocity graph compared to an
acceleration graph??

Joel

One small detail has escaped you, possibly because of an inadequacy in my
description. This event of the 'wind up' in the pulley scaffolding and the
subsequent vibrations occurs _after_ the end of a run _as_ the cart is
being brought to a stop (when the 'wind up") occurs and _after it becomes
stationary_ and is being held in place (when the subsequent vibrations
occur). (The cart is being held down and the pulley scaffolding is
vibrating.)

The force experienced by the probe is not ever really the _net_ force,
especially when the cart is being restrained or experiencing significant
friction. It's only close to the net force when the cart is rolling along
a horizontal ramp with a force pulling (or pushing) on the probe attached
to the cart. Strictly, the force graph is a graph of the force experienced
by the force probe. This is essentially a one-dimensional device. The
force the probe detects is also 'experienced' by the cart if the probe is
somehow strapped to the cart.

That friction is just part of the background of the students' experience
(not explicitly considered) becomes very clear when you watch how
non-trivial it appears to be for students to translate their own
conclusions from low friction situations to situations in which significant
friction is added to the carts.

So, the force graph not really a graph of the net force and I did really
mean the acceleration graph.

The differences between our idealizations and the practicalities of real
situations...

Joel, hope this helps.

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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