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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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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):
The position of the pendulum at three different times:
- 59.4s, 6 in.
- 59.91s, 16in.
- 60.45s, 20in.
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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):
I would try to calculate the speed of the pendulum to see at which points during its travel that it is the fastest and at which point is it at its slowest. I would estimate that my data is accurate by up to about 5 centimeters because the pendulum was so thick that I could not determine the exact position it was at and the measuring tape was too far from the camera to make a more accurate estimate. The clock time for the first and third observations was taken directly from the computer screen so I would say they are accurate up to the 0.01s but the second observation of the time was inhibited by the stick so I was not able to get a very accurate recording 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):
By calculating the speed the tape was going at the beginning of the incline by dividing the distance it rolled by the time it took to get there and then comparing it to the speed the tape was rolling at the end of its run by doing the same calculation for the two new points you can see whether it was going faster at the beginning or at the end.
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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 calculate the speed the pendulum was going at the beginning by picking a position along the measuring tape towards the top and then dividing that number by how long it took the pendulum to get there and then doing the same calculation for a position towards the bottom of the ramp to compare whether the pendulum was going faster at the top or the bottom.
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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):
By measuring the speed of the pendulum at many different positions you can determine when the pendulum speeds up and when it slows 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):
By following the same method I mention above and then comparing the speeds at different intervals you could see if it is speeding up or slowing down.
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