🚀 go-pugleaf

I have done exactly this lab in my classes this semester.  We used the
aircart instead of a troll, but it was still irritating.  First thing I
would check is to see how many times the computer is sampling the force
sensor.  We used differen software (Motion Plotter) and it isn't fast
enough to account for less than 10% error or so.  You should be able to
see how many times it is sampled some how, if only counting the dots on
the screen.
        Second, it turns out that the mass of the bungee cord seems to be
more relevant than one might think.  It can be calculated only to about
50% accuracy as I figure, but sometimes it helps to add that in to the
theoretical value.  You might check this by comparing the mass of the
string to the mass of the troll, if they are different by less than 90%,
I would consider this a major source of error.
        Try the sampling rate and the cord momentum and see what you get.
Brandon Kovach
Drury College
Springfield, Mo


On Sat, 3 Dec 1994, Lowell Herr wrote:

> I am trying an impulse - change in momentum experiment with my students
> and we are getting a 30% error.  Let me explain.
> 1.  By sending a glider across an air track, a rubber bumper on the glider
> collides and rebounds off a force probe.  Measuring the area under the
> F*delta t graph (using MacMotion software) we can compare the impulse with
> the change in momentum of the glider.  This works within a few %.  No
> problems.
>
> 2.  Now we set up a Bungee Jumper using a Troll that is pushed off a
> platform with a long rubber band attached to the Troll's ankle and a force
> probe.  The velocity of the falling Troll is calculated just before the
> rubber band begins to stretch.  Knowing the change in momentum of the
> Troll during the fall and related rebound, one can calculate the change in
> momentum.  In this experiment, the impulse comes out 30% higher than the
> change in momentum.  I have gone through the experiment several times and
> cannot account for such a large difference.  The impulse curve generated
> is very symmetrical and consistent.  The Troll I am using has a mass of
> 0.23 kg placing a good force on the probe as it falls 73 cm (using
> center-of-mass measurements).  I have the probe locked down quite well so
> as to reduce vibrations.  I have calibrated the probe several times
> thinking that might be a problem.  Any suggestions?  The impulse is
> consistently 30% - 35% higher than the change in momentum.
>
> Lowell Herr
> Project PHYSLab
> The Catlin Gabel School
> Portland, OR 97225
>