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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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 the roll of tape. The clock times for the three stops were 20.234 s, 20.671 s, and 21.109 s. The positions, respectively, were 2 in, 12 in, and 21 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:
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):
If I were to use these data for calculations, I could, reasonably accurately, determine whether or not the roll of tape was speeding up or slowing down. I could do this by calculating the speed during each interval that I timed. I would do this by dividing the number of inches travelled by the number of seconds that passed to get the tapes speed in inches per second. After obtaining these three speeds, I would be able to see whether it is speeding up or slowing down by comparing the values. I think I could determine the positions accurately to within 1 inch because although I cannot see the actual numbers on the measuring tape, I can see the mark for 1 foot and I can count each way from there to determine the number of inches. I do not think I can be more accurate than within 1 inch, however, because of the quality of the video; it makes it hard to determine how close to each inch marking the tape actually is. I think I can determine the time accurately to within .001 seconds because the timer gives values that accurate and when I stop the video, it stops on an exact time.
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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):
I could use this data to determine the speed during each interval by dividing the distance travelled (in inches) by the time elapsed (in seconds) and get the speed in inches per second. I could then compare the speeds in each interval to determine which is the greatest and determine whether or not the tape is speeding up as it rolls down the incline.
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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):
I could use position and time to determine whether the pendulum is speeding up or slowing down in the same way I used them to determine the behavior of the roll of tape. I would find the speed of the pendulum in different intervals measured in inches per second and compare them to each other to determine which is the greatest.
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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):
To determine the location that a pendulum starts slowing down, I would take multiple measurements of position and time. Ideally, I would want to take many measurements of the same length. I think the best way to do this would be to measure the time at every inch so I could calculate the speed at a large number of intervals. I would then graph the results (position vs. speed) to see how the speed changes. Then, using the graph, I could determine where the pendulum starts slowing down because it would be at the maximum point of the graph.
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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):
I would again take measurements of time at every inch and with this information, determine the speeds at each position. Then using the information of position and speed, I would graph my results (position vs. speed). From the graph, I would be able to determine whether the roll of tape is speeding up at a constant, increasing or decreasing rate. If the graph is a straight line, it would be speeding up at a constant rate. If the graph is concave down, it would be speeding up at a decreasing rate and if the graph is concave up, it would be speeding up at an increasing rate.
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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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30 minutes
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