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Unformatted text preview: obtaining results is the only reliable way to take data.
How else can you know if something has gone wrong? This happens so often that it is given
a name (Murphy’s Law). It is also a good way to make sure you have learned something, but
only if you stop to think about the discrepancies or similarities between your prediction and
In order to enter your prediction into the computer, you first need to decide on your
coordinate axes, origin, and scale (units) for your motion. Record these in your lab journal.
Next you will need to select the generic equation, u(t),
which describes the graph you expect for the motion
along your x-axis seen in your video. You must choose
the appropriate function that matches the predicted
curve. The analysis program is equipped with several
equations, which are accessible using the pull-down
menu on the equation line. The available equations
are shown to the right. You can change the equation to one you would like to use by clicking on the arrows to the
left of the equation
After selecting your generic equation, you next need to enter your best approximation for the
parameters A and B and C and D where you need them. If you took good notes of these
values during the filming of your video, inputting these values should be straightforward.
You will also need to decide on the units for these constants at this time.
Once you are satisfied that the equation you selected for your motion and the values of the
constants are correct, click "Accept" in the Main Controls. Your prediction equation will then
show up on the graph on the computer screen. If you wish to change your prediction simply
repeat the above procedure. Repeat this procedure for the Y direction.
217 APPENDIX: SOFTWARE To collect data, you first need to identify a very specific point on the object whose motion you
are analyzing. Next move the cursor over this point and click the green ADD Data Point
button in Data Acquisition control box. The computer records this position and time. The
computer will automatically advance the video to the next frame leaving a mark on the point
you have just selected. Then move the cursor back to the same place on the object and click
ADD Data Point button again. So long as you always use the same point on the object, you
will get reliable data from your analysis. This process is not always so easy especially if the
object is moving rapidly. The data will automatically appear on the graph on your computer
screen each time you accept a data point. If you don’t see the data on the graph, you will
need to change the scale of the axes. If you are satisfied with your data, choose Quit Data Acq
from the controls
FITTING YOUR DATA
Deciding which equation best represents your data is the most important part of your data
analysis. The actual mechanics of choosing the equation and constants is similar to what you
did for your predictions.
First you must find your data on your graphs. Usually, you can find your full data set by
using the Autorange buttons to the left of the graphs.
Secondly, after you find your data, you need to determine the best possible equation to
describe this data. After you have decided on the appropriate equation, you need to
determine the constants of this equation so that it best fits the data. Although this can be
done by trial and error, it is much more efficient to think of how the behavior of the equation
you have chosen depends on each parameter. Calculus can be a great help here.
Lastly, you need to estimate the uncertainty in your fit by deciding the range of other lines
that could also fit your data. This method of estimating your uncertainty is described in the
appendix Accuracy, Precision and Uncertainty. Slightly changing the values for each
constant in turn will allow you to do this quickly. For example, the X-motion plots below
show both the predicted line (down) and two other lines that also fit the data (near the
circles). 218 APPENDIX: SOFTWARE After you have found the uncertainties in your constants, return to your best-fit line and use
it as your fit by selecting Accept x- (or y-) fit in the Program Controls panel.
These directions are not meant to be exhaustive. You will discover more features of the video
analysis program as you use it. Be sure to record these features in your lab journal. 219 APPENDIX: SOFTWARE MAGNETLAB - MEASURING CONSTANT MAGNETIC FIELD
Before you begin, you should ensure that you have read the relevant sections of Appendix A
to familiarize yourself with the equipment.
The software package that works in tandem with your magnetic field sensor is written in
LabVIEW™. It allows you to measure and record magnetic field strength as a function of a
number of different variables.
After logging into the computer, execute the application by double clicking the
“MAGNETLAB” icon located in the PhysLab folder on the desktop.
Before you start using the program, you
should take a moment to identify several
key elements. The two most important of
these are the Command Panel, shown to the
right, and the Guide Box, shown bel...
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This document was uploaded on 02/23/2014 for the course MANAGMENT 2201 at University of Michigan.
- Spring '14