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PHY 241
Your 'cq_1_00.1' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
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The videos
There are four short videos, all of the same system. The smaller files are around 500 kB and will download faster than the larger files, which are about 4
times that size (about 2 mB or 2000 kB), but the larger files are a bit better in quality. If you have a fast connection any of these files should download
fairly quickly. Video 1 and Video 2 probably contain the best information; Video 4 is the shortest.
The quality of these videos is not that great, and that is deliberate. These are medium-definition videos, taken with a camera that doesn't have a
particularly high shutter speed. It's not important here to even know what a shutter speed is, but the effect of the slow shutter speed is to cause images of
moving objects to be blurry.
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All data in any science is in effect 'blurry'--there are limits to the precision of our measurements--and we start off the course with images that have
obvious imperfections. We will later use images made with a high-definition camera with a fast shutter, where imperfections, though still present, are
difficult to detect.
Video 1 (smaller file) Video 1 (larger file)
Video 2 (smaller file) Video 2 (larger file)
Video 3 (smaller file) Video 3 (larger file)
Video 4 (smaller file) Video 4 (larger file)
View these videos of a white roll of tape rolling down an incline next to a dark swinging pendulum, using Windows Media Player or a commercial media
player. By alternately clicking the 'play' and 'pause' buttons you will be able to observe a series of positions and clock times.
The measuring tape in the video may be difficult to read, but it is a standard measuring tape marked in feet and inches. At the 1-foot mark, a little to the
left of the center of the screen, there is a black mark on the tape. If you want to read positions but can't read the inches you can count them to the right and
left of this mark. You can estimate fractions of an inch. You don't need to write anything down; just take a good look.
Begin by forming an opinion of the following questions; for the moment you may ignore the computer screen in the video. You don't have to write
anything down at this point; just play with the videos for a couple of minutes and see what you think:
think it is intresting how one person can have three hands and yet none of them can be seen in the clip--WOW
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Is the tape speeding up or slowing down?
*Speeding up
Is the pendulum speeding up or slowing down?
*it seems to be slowing due to relative velocities
Which speeds up faster, the tape or the pendulum?
*They are equally fast until the pendulum reaches the equalibrium position and begins to slow.
What is going to limit your ability to precisely measure the positions of these objects?
view of ruler, the optical closeness. We have no true idea of the clock time.
The computer in the video displays the running 'clock time', which is accurate to within something like .01 second. Think about how the information on
this screen can help answer the above questions.
now we can find /w `ds and `dt we can find vAve, we know v0 => we can determine the acceleration.
You don't have to think about the following right now, so I'm going to make it easy to ignore by putting it into small type. There is a parallax issue here.
You don't even have to know what this means. But if you do, and if you want the information, here it is:
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The measuring tape is pretty much parallel to the paths of the pendulum and the tape roll, about 5 inches further from the camera than the path of the
pendulum, and the path of the ball is about halfway between the two. The camera is about 5 feet away from the system.
The problem:
You don't have to actually do so, but it should be clear that if you wished to do so, you could take several observations of positions and clock times. The
main point here is to think about how you would use that information if you did go to the trouble of collecting it. However, most students do not answer
these questions in terms of position and clock time information. Some students do not pause the video as instructed. To be sure you are thinking in terms
of positions and clock times, please take a minute to do the following, which should not take you more than a couple of minutes:
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Pick one of the videos, and write down the position and clock time of one of the objects, as best you can determine them, in each of three different
frames. The three frames should all depict the same 'roll' down the ramp, i.e. the same video clip, at three different clock times. They should not include
information from two or more different video clips.
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For each of the three readings, simply write down the clock time as it appears on the computer screen, and the position of the object along the meter
stick. You can choose either object (i.e., either the pendulum or the roll of tape), but use the same object for all three measurements. Do not go to a lot of
trouble to estimate the position with great accuracy. Just make the best estimates you can in a couple of minutes.
Which object did you choose and what were the three positions and the three clock times?
answer/question/discussion: ->->->->->->->->->->->-> scussion (start in the next line):
The tape
59.25 = dt, 2.25 in =dx
dt=59.69 s, 12 in =dx
59.80 s = dt, 17.125 in :)
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In the following you don't have to actually do calculations with your actual data. Simply explain how you would use data of this nature if you had a
series of several position vs. clock time observations:
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If you did use observations of positions and clock times from this video, how accurately do you think you could determine the positions, and how
accurately do you think you would know the clock times? Give a reasonable numerical answer to this question (e.g., positions within 1 meter, within 2
centimeters, within 3 inches, etc; clock times within 3 seconds, or within .002 seconds, or within .4 seconds, etc.). You should include an explanations of
the basis for your estimate: Why did you make the estimate you did?
answer/question/discussion: ->->->->->->->->->->->-> scussion (start in the next line):
t within .01 sec, dx within .5 in
I could see the timers values clearly they seemed acurate out to three decimal places hence if one is an estimate the acuarcy is .01. I used the same
reasoning for the `dx.
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How can you use observations of position and clock time to determine whether the tape rolling along an incline is speeding up or slowing down?
answer/question/discussion: ->->->->->->->->->->->-> scussion (start in the next line):
we could determine the `dx vs. `dt to see whether the tape is speeding up or not.
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How can you use observations of position and clock time to determine whether the swinging pendulum is speeding up or slowing down?
answer/question/discussion: ->->->->->->->->->->->-> scussion (start in the next line):
the same way as above
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Challenge (University Physics students should attempt answer Challenge questions; Principles of Physics and General College Physics may do so but
it is optional for these students): It is obvious that a pendulum swinging back and forth speeds up at times, and slows down at times. How could you
determine, by measuring positions and clock times, at what location a swinging pendulum starts slowing down?
answer/question/discussion: ->->->->->->->->->->->-> scussion (start in the next line):
you could find the mid t or mid `dx for the pendulum this is where the pendulum should slow at equilibrium.
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Challenge (University Physics students should attempt answer Challenge questions; Principles of Physics and General College Physics may do so but
it is optional for these students): How could you use your observations to determine whether the rate at which the tape is speeding up is constant, increasing
or decreasing?
answer/question/discussion: ->->->->->->->->->->->-> scussion (start in the next line):
knowing that the tape starts from rest: the rate of speed [increasing or decreasing] is = `ds / `dt * 2 / 'dt =
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Check to see that you have followed the instructions:
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The instructions told you to pause the video multiple times. It appears that some students are not following this instruction.
If you haven't used the 'pause' and 'play' buttons on your media player, you should go back and do so.
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The questions are phrased to ask not only what you see when you play the video, but what you see when you pause the video as instructed, and what
you think you could determine if you were to actually take data from the video. You aren't asked to actually take the data, but you need to answer how you
would use it if you did.
It's OK if you have given more general descriptions, which are certainly relevant. But answers to the questions should include an explanation of how
you could use the series of position and clock time observations that are may be observed with this video.
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The questions also ask how much uncertainty there would be in the positions and clock times observable with this specific video. Different people will
have different answers, and some reasonable answers might vary from one clip to the next, or from one part of a clip to another. However the answers
should include a reasonable quantitative estimate (i.e., numbers to represent the uncertainty; e.g., .004 seconds of uncertainty in clock times, 2 inches in
position measurements. Use your own estimates; neither of these example values is necessarily reasonable for this situation). You should also explain the
basis for your estimate: why did you make the estimate you did?
You should have estimated the number of seconds or fraction of a second to within which you think the time displayed on the computer screen might be
accurate (e.g., is it accurate to within 10 seconds of the actual clock time, or to within 1 second, within .1 second, maybe even within .01 or .001 second).
You might not yet know enough about the TIMER to give an accurate answer, but give the best answer you can.
You should also indicate a reasonable estimate of the number of inches or fraction of an inch to within which you could, if asked, determine the position
of each object.
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1 hr.
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Vista would not play the video files for me even after downloading the videos. I had to download the VLC player from online. This took about 20 extra min.
Looks good.
&#See any notes I might have inserted into your document, and before looking at the link below see if you can modify your solutions. If there are no notes, this does not mean that your solution is completely correct.
Then please compare your old and new solutions with the expanded discussion at the link
Solution
Self-critique your solutions, if this is necessary, according to the usual criteria. Insert any revisions, questions, etc. into a copy of this posted document. Mark any insertions with &&&& so they can be easily identified.If your solution is completely consistent with the given solution, you need do nothing further with this problem. &#