Phy 231
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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Copy the problem below into a text editor or word processor.
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• Include the text of the entire problem, starting with the words 'The Problem:'.
• You might prefer for your own reasons to use a word processor (for example the formatting features might help you organize your answer and explanations), but note that formatting will be lost when you submit your work through the form.
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You should enter your answers into this copy using the text editor or word processor. Enter your response to each question following the answer/question/discussion **************: prompt.
You will then copy-and-paste the document, which will include the questions and your answers, into the box below, and submit.
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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 **************:
I picked the pendulum to be my object of interest and for the first three frames I tried to stop the clock when the pendulum was just about even with the 1 ft mark, before, and after on the tape measure. For video 1 the clock time was 59.796 when it was even with the 1’, about 2” before the 1’ mark the time was 59.578, and about 5” past the 1’ mark the time was 60.015. Also at the half way point the tape is beginning to pass the pendulum.
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 **************:
From these videos I think you could accurately measure the position within an inch because it really is hard to see and be able to stop the video where you want. Also as far as the clock time goes I think that you could be within .1 of the actual from where you are interested in. I was watching the videos and it takes about a second to reach the bottom so I think if you are pretty close to say the 1 ft mark then you would be within one tenth of a second off or within an inch. Also, the tape looks like it is 2 ft long and it hits the bottom in one second, so the tape is traveling about 24” per second
• 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 **************:
Based on the pendulum you can see that the tape is exceeding the speed of that which would mean that it is speeding up as it is rolling down. Also, it reaches the end before the pendulum does based on the clock times.
• 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 **************:
I would think that the pendulum is slowing down because in one of the videos the pendulum starts to swing back so therefore as it is approaching the end it has to be slowing down because it is about to swing back the other way
• 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 **************:
I would say when the pendulum reaches the point where it is completely vertical or it is the z-axis that when it begins to go up at that point it is slowing down and when it reaches the point where it changes direction it will automatically begin to speed up until it hits the z-axis again.
• 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 **************:
By looking at the time it took to reach the half way point you can see that it takes just slightly less time for it to reach the end then it does half way. Because of this I would think the tape has to be speeding up
Check to see that you have followed the instructions:
• 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.
• 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.
• 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 hour
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&#Your work looks good. Let me know if you have any questions. &#