Re: Help!!An interesting acceleration graph

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

> To all:
> My high school physics students have been looking at Newton's laws using the
> Vernier Motion Detector and Force probe on the Macintosh Computer. We're
> running Pasco Carts on their aluminum track and hanging a weight using their
> super pulley. This should give us really nice graphs, but...
>
> Our acceleration graph goes up, comes back down then levels off. eg, there's
> a bit of a bulge, just as the cart is released. We're careful about hands
> and other things that might interfere with the motion detector, but I have
> no explanation for this. Has anyone seen this? Can anyone help?
>
> arrgh
>
> Don McQuarrie (not the chem text author)

Don:
I don't know whether this is an answer to your 'problem' or not, but in the
very early days (1986 or so) when this stuff was ONLY on Apple II, the
software did not handle making acceleration graphs well when the cart was
already moving.  I got into the habit of having the students hold
everything very still and waiting to release until a second or two after
the detector started making audible tics.

We do this same thing as you with the same apparatus as far as I can tell,
but we are not seeing this effect.  We instruct the students to gently
touch the RAMP with a finger in front of the cart, holding the cart still
and not with so much weight that the track sags (which would cause a
'bounce' in the track).  They have a tendency to merely put a finger ON the
cart to hold it still, but this is virtually impossible to do (hold it
still and start it cleanly this way, that is).  Then they are instructed to
move their finger quickly away from the cart TOWARD or IN the direction the
cart is going to move (ONLY), NOT to pull their fingers out to the side of
the ramp perpendicular to the potential travel, and then get themselves and
their hands out of the beam before the cart goes by them.  It helps for
them to be already standing 'down beam' from the cart in the first place
when they release the cart.

We have another student be the "catcher" who stays off the ramp and the
table during the run, but catches (traps is a better word) the cart to a
stop and holds it there for a second or two until the graph is complete.
If the catcher is resting arms or elbows on the table holding the ramp, we
usually see the catcher's anticipation of the catch (getting ready to
catch) in the motion graphs as well as the force graph, so the catcher HAS
to stay off the ramp and table until the instant of the catch.  Having this
period of no motion both before the release and after the catch, creates
portions of the graph (the start and the catch) which are quite
instructive, especially for motions which involve shoves up the track.  If
you are not having them study these, then you are missing some VERY
effective experiences.

Another thought.  Depending on the nature of your tables, maybe regardless
of the nature of the tables, EVERYONE should be free and clear of the table
on which the ramp rides.  Our tables are pretty massive, but they can rock
and shift.  If people have elbows, etc. on the table and they shift their
weight (as they reach or sit back from hitting the return key or as they
watch the cart go by, etc.) then this shows up in the graphs, because the
table, ramp and detector move underneath the cart (the consequences of a
low friction cart).

We have our string go UNDER the pulley to a pulley above and then down to
the hanging mass.  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.

Now that I think carefully about our experience and your description, I
think I know what you are talking about and we see this too.  Generally I
find coaching them at the technique of releasing the cart can reduce this
initial bulge (sometimes a spike) in the acceleration back to what the
graph should be - a rise from zero to a constant acceleration.  Most lab
groups can get good runs with a little practice, some still have somewhat
of a bulge at the beginning of the runs.  Again, I don't think I saw this
as much before we went to the low-friction carts.

Good luck.  Your students can get good graphs, I'm sure.

Dewey

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++