Phy121 *&$*&$
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.
• 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: ->->->->->->->->->->->-> :
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?
I looked at distances and clock times of both the tape and pendulum. If I were to give positions and clock times of either item my personal estimates would not be very accurate. Part of this is because when I went to pause the video it always seemed to pause in almost the same spot. I think this is partly because it took my computer that long to respond to what I was asking it to do. My guesses on length may be a bit closer that the actual time as the distances I tried to focus on were the start (around 0-2 inches) the center (around 11-13 inches) and then end (maybe around 22-24 inches). Times would be within several tenths of a second. I chose the locations to estimate times because I thought they would be the easiest to record. The same for the times. When looking at my times for each of my distances, my first time appeared to be around 59, the second time measurement was around 60.6 and the final time measurement was around 61.7
answer/question/discussion: ->->->->->->->->->->->-> :
• 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?
You can use these observations to see if the tape is gaining or losing time over the set distances you checked for time and distance traveled. If the tape appears to have moved faster in the second length/time period observed then one could assume it is speeding up. If it took longer the second length/time period observed then one could assume it was slowing down.
When I started the video, the tape was at the top of the incline and the time was almost 59 seconds at the start. In the center (around 1 ft.) it was just under 60 and at the end it was just over 60 (60.23). I think that the tape was speeding up because it was gaining more momentum as it traveled.
answer/question/discussion: ->->->->->->->->->->->-> :
• How can you use observations of position and clock time to determine whether the swinging pendulum is speeding up or slowing down?
You could use almost the same method to determine whether or not the pendulum was slowing down or speeding up as you did for the tape. However, since the pendulum travels in a complete cycle you would need to record how fast it went for the first half of the cycle and compare this with the second half of the cycle traveled. You could even go so far as to break up each half of the cycle to see which part of that half cycle the pendulum speeds up and slows down in.
When the pendulum started, the clock was right around 59. It was around 60.6 in the center (around 1 ft) and then stopped at around 62 seconds. Based on this data I would say that the pendulum was slowing down at it made half a cycle. I would expect it to speed back at when it began to complete the other half of the cycle and then slow down again. I think this also goes back to momentum.
answer/question/discussion: ->->->->->->->->->->->-> :
• 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: ->->->->->->->->->->->-> :
• 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: ->->->->->->->->->->->-> :
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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20 -25 minutes.
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Submitted on 1/22 2
&#Very good work. Let me know if you have questions. &#
&#Your work is mostly correct, and I believe you will understand everything after reading the document in the link below. You will be directed to submit a revision; however unless you have questions or comments, the revision is not necessary. Just be sure you understand all the important details in the document. &#