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Phy 121
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.
** CQ_1_00.1_labelMessages **
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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.
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:
Is the tape speeding up or slowing down?
Is the pendulum speeding up or slowing down?
Which speeds up faster, the tape or the pendulum?
What is going to limit your ability to precisely measure the positions of these objects?
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.
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:
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:
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.
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: ->->->->->->->->->->->-> (start in the next line):
I chose to observe the roll of tape in the second video.
P1: 7
CT: 40.578
P2: 14
CT: 40.906
P3: 21
CT: 41.453
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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:
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: ->->->->->->->->->->->-> (start in the next line):
Looking at my data, I noticed that the position of the tape roll was in positions in increments of 7. I.E. my first reading was 7 at 40.578, my second was 14 at 40.906,
and the last was 21 at 41.453. If I were able to slow the playback speed, I might be able to calculate the actual time it took the tape to roll each 7 increment.
For each of my observations, I had at least one uncontrolled variable that detracted from the accuracy of my readings. The first observation I guessed the tape roll to
be a 7, though if the tape were not as blurry I may have been able to find a reading of 6.89. The last measurement I had to estimate completely because the
pendulum was obstructing the view of the front of the tape.
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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: ->->->->->->->->->->->-> (start in the next line):
By taking the difference of the next reading from the first reading, then taking the diff. of the final reading from the second reading, I find that to travel the first 7
inches took 0.328 sec. To travel the second 7 inches (14-21), the tape took 0.547sec. This shows the tape is slowing as it travels further down the incline. Again, since
my observation of distance on the tape measure is blurred, and I had to guess at the exact position, I may have found that instead of the first position being 7 it was
actually 6.75, and so on. I don't think this would have made a huge difference in the speed of the roll of tape though.
The best way to show the speeding up of the roll of tape is by calculating the inches per second speed at each clock reading. I divided the position in inches by the
clock time to calculate in./sec.
Reading Speed
1 .17251/sec
2 .342248/sec
3 .506598/sec
With each consecutive reading, the speed is increasing. I can see this because each consecutive reading is more inch/sec than the previous.
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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: ->->->->->->->->->->->-> (start in the next line):
The speed of the pendulum for one cycle could be figured by observing the position of the pendulum at its resting position, halfway through the swing, and at the
zenith of its swing, and noting the times for each of these positions. After calculating the inch/sec of each of these positions, the clock readings should be taken again
at the same points on the measuring tape. The speeds of the first set should be compared to the speeds of the second set of data. This would show if the pendulum
was faster on the first swing than the second swing.
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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: ->->->->->->->->->->->-> (start in the next line):
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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: ->->->->->->->->->->->-> (start in the next line):
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&#Good responses. Let me know if you have questions. &#