Phy 201
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 to measure the movement of the roll of tape from Video 1. The following data: the first column is the clock time in seconds and the second column is the position in inches.
59.359, 3
59.687, 12
60.015, 22
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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 explanation of the basis for your estimate: Why did you make the estimate you did?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
The positions would probably be within 3 centimeters of the actual position. This estimate is accounting for the uncertainty of the film and the measuring tape. The roll of tape looked to be within the range of the position given, but could have been about an inch away, so the uncertainty would be about 3 centimeters. The clock time would be within about .005 of the actual time. Since there are three decimal place on the clock, the last certain one would be the hundredths place. The thousandths place is the uncertain one, so the time could be within .005 of the 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):
You can find the time when the tape is at the middle of the incline. Then you use the initial time and then end time to determine the time it take for the tape to roll down the first half of the incline and the second half. If the second half is faster, then you know that that tape has gained speed as it rolled.
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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):
You can observe the time it takes for the pendulum to complete a cycle. Then you observe many other cycles individually (based on the clock time because you do not want an average—it won’t tell you anything). Once you have the times for several different cycles, you can compare the times. If the times for a complete cycle increase, the pendulum is slowing down.
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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):
You have to have time intervals for many positions of the pendulum during one cycle. Then you find the speed of the pendulum from one point to the next. When the pendulum is speeding up and is fastest, the times will be the lowest and when it begins slowing down, the times will get longer because it is taking longer for the pendulum to move to the next point. If you can get the average speed for many small intervals, then you can graph the data and look at the curve produced (it will be the first derivative, for velocity). When the graph crosses the x-axis from positive to negative it means the velocity has changed directions and that the pendulum is beginning to slow down.
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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):
You can plot the speed data on a graph, which is an equivalent to the first derivative graph. Most likely the curve would be similar to an exponential graph, because the roll of tape continues to speed up. Then you take the derivative of that graph, which would be the second derivative (for acceleration); it would be another exponential graph, which would illustrate that the acceleration starts slow, but then increases as the roll of tape moves down the incline.
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30 minutes
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6/4 6
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