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course Phy 201
September 26 around 10:45I used this Submit Work Form because the link provided in the Pearl Pendulum Lab Assignment tab says it is currently unavailable.
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The simple device used in this experiment can serve as an accurate timing device when the 'beats' of the pendulum are synchronized with two events separated by a consistent time interval. Observations of this system are consistent with the observed and theoretically predicted behavior of pendulums. Most students report that the experiment takes around an hour, with a range from 30 minutes to 2 hours, and in a few cases longer.
The Pearl Pendulum as shown below a bead (sometimes called a 'pearl', as the bead used in the original version was a fake plastic pearl) on a string, attached to bolt glued to the top of a metal bracket, using a magnet to 'clamp' the string (in most current versions of the apparatus the bolt glued to the top of the bracket, which proved to be unhelpful, is not included).
You will need to construct the pendulum using the small bead and thin copper wire packed in your lab materials package. In the Spring 2010 version the bead and the wire were taped to the bracket, or packed in a separate bag with another small object.
The wire is formed into a loop with the two ends protruding, and threaded through the bead.
The ends are pulled through forming a small loop at the top.
The protruding ends are twisted together then flattened against the bottom of the bead.
The above pictures were actually of a steel ball and a thicker wire. The bead and wire you have in your kit look something like this:
When suspended from the pendulum bracket by a thread the system might look something like the picture below. If the pendulum is pulled back and released, it will bounce back to the bracket, rebound, and repeat its motion a number of times.
However note that in this picture the bracket is resting on end with the bolt glued to it; the bracket is not vertical.
The pearl appears to hanging in its equilibrium position, with a little space between it and the bracket.
As you will soon see, if the bead is just barely touching the bracket when it hangs at its equilibrium position, the rhythm of the bouncing pendulum will remain constant.
The bead is referred to below as the 'pearl'.
When the pearl is released it swings back to the bracket, bounces off the swings back again, repeatedly striking the bracket. The magnet can be used to clamp the thread so the length of the pendulum remains constant.
If you have just a plain bracket then you simply tilt the bracket in order to achieve a constant rhythm, as described below.
You should set the system up and allow the pearl to bounce off the bracket a few times. The bracket should be stationary; the pendulum is simply pulled back and released to bounce against the bracket.
Note whether the pearl strikes the bracket more and more frequently or less and less frequently with each bounce. If the pearl does not bounce off the bracket several times after being released, it might be because the copper wire below the pearl is getting in the way. If necessary you can clip some of the excess wire (being careful to leave enough to keep the bead from falling through).
If the bracket is tilted back a bit, as shown in the next figure below, the pearl will naturally rest against the bracket. Tilt the bracket back a little bit and, keeping the bracket stationary, release the pendulum.
Listen to the rhythm of the sounds made by the ball striking the bracket.
Do the sounds get closer together or further apart, or does the rhythm remain steady? I.e., does the rhythm get faster or slower, or does it remain constant?
Repeat a few times if necessary until you are sure of your answer.
Insert your answer into the space below, and give a good description of what you heard.
Your response (start in the next line):
When the bracket is tilted back, the sounds of the pearl striking the bracket gets closer together. Therefore, the rhythm gets faster. I heard the pearl strike the bracket at an increasing rate.
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If the bracket is tilted forward a bit, as shown in the figure below, the pearl will naturally hang away from the bracket. Tilt the bracket forward a little bit (not as much as shown in the figure, but enough that the pearl definitely hangs away from the bracket). Keep the bracket stationary and release the pendulum. Note whether the pearl strikes the bracket more and more frequently or less and less frequently with each bounce.
Again listen to the rhythm of the sounds made by the ball striking the bracket.
Do the sounds get closer together or further apart, or does the rhythm remain steady? I.e., does the rhythm get faster or slower, or does it remain constant?
Repeat a few times if necessary until you are sure of your answer.
Insert your answer into the box below, and give a good description of what you heard.
Your response (start in the next line):
When the bracket is tilted forward, the sound of the ball striking the bracket occurs further apart. Therefore, the rhythm gets slower. I heard the pearl strike the bracket at a decreasing rate.
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If the bracket is placed on a perfectly level surface, the pearl will hang straight down, just barely touching the bracket. However most surfaces on which you might place the bracket aren't perfectly level. Place the bracket on a smooth surface and if necessary tilt it a bit by placing a shim (for a shim you could for example use a thin coin, though on most surfaces you wouldn't need anything this thick; for a thinner shim you could use a tightly folded piece of paper) beneath one end or the other, adjusting the position and/or the thickness of the shim until the hanging pearl just barely touches the bracket. Pull the pearl back then release it.
If the rhythm of the pearl bouncing off the bracket speeds up or slows down, adjust the level of the bracket, either tilting it a bit forward or a bit backward, until the rhythm becomes steady.
Describe the process you used to make the rhythm steady, and describe just how steady the rhythm was, and how many times the pendulum hit the bracket..
Your response (start in the next line):
When the bracket was placed on a perfectly level surface, the rhythm remained steady. I had to place a folded piece of paper to achieve a level surface. The pendulum hit the bracket six times at a steady rate.
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On a reasonably level surface, place one domino under each of the top left and right corners of your closed textbook, with the front cover upward. Place the bracket pendulum on the middle of the book, with the base of the bracket parallel to one of the sides of the book. Release the pendulum and observe whether the sounds get further apart or closer together. Note the orientation of the bracket and whether the sounds get further apart or closer together.
Now rotate the base of the bracket 45 degrees counterclockwise and repeat, being sure to note the orientation of the bracket and the progression of the sounds.
Rotate another 45 degrees and repeat.
Continue until you have rotated the bracket back to its original position.
Report your results in such a way that another student could read them and duplicate your experiment exactly. Try to report neither more nor less information than necessary to accomplish this goal. Use a new line to report the results of each new rotation.
Your response (start in the next line):
After placing the dominoes under my book, I put the bracket in the middle of the book with the base parallel to the sides. The front of the bracket was facing towards the top of the book, or up the incline. When I released the pendulum, the sounds continued to get closer together. With each 45 degree counterclockwise rotation, the sounds of the pendulum began to continuously get further apart. Once the bracket rotated 180 degrees, it was facing down the incline. At this point, the sounds of the pendulum were significantly further apart than the original position. Then, as I continued to rotate 45 degrees counterclockwise for the remaining 180 degrees, the sounds of the pendulum began to get closer together until I reached the original position.
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Describe how you would orient the bracket to obtain the most regular 'beat' of the pendulum.
Your response (start in the next line):
The bracket obtained the most regular beat of the pendulum when it was placed at a 90 degree angle to the side of the book.
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Orient the bracket in this position and start the TIMER program. Adjust the pendulum to the maximum length at which it will still bounce regularly.
Practice the following procedure for a few minutes:
Pull the pendulum back, ready to release it, and place your finger on the button of your mouse. Have the mouse cursor over the Click to Time Event button. Concentrate on releasing the pendulum at the same instant you click the mouse, and release both. Do this until you are sure you are consistently releasing the pendulum and clicking the mouse at the same time.
Now you will repeat the same procedure, but you will time both the instant of release and the instant at which the pendulum 'hits' the bracket the second time. The order of events will be:
click and release the pendulum simultaneously
the pendulum will strike the bracket but you won't click
the pendulum will strike the bracket a second time and you will click at the same instant
We don't attempt to time the first 'hit', which occurs too soon after release for most people to time it accurately.
Practice until you can release the pendulum with one mouse click, then click again at the same instant as the second strike of the pendulum.
When you think you can conduct an accurate timing, initialize the timer and do it for real. Do a series of 8 trials, and record the 8 time intervals below, one interval to each line. You may round the time intervals to the nearest .001 second.
Starting in the 9th line, briefly describe what your numbers mean and how they were obtained.
Your response (start in the next line):
0.602
0.574
0.581
0.576
0.560
0.531
0.503
0.483
These numbers indicate the 8 times that the pendulum hit the bracket. They were obtained by using the TIMER program. The Click to Time button was clicked at each strike of the pendulum.
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Finally, you will repeat once more, but you will time every second 'hit' until the pendulum stops swinging. That is, you will release, time the second 'hit', then time the fourth, the sixth, etc..
Practice until you think you are timing the events accurately, then do four trials.
Report your time intervals for each trial on a separate line, with commas between the intervals. For example look at the format shown below:
.925, .887, .938, .911
.925, .879, .941
etc.
In the example just given, the second trial only observed 3 intervals, while the first observed 4. This is possible. Just report what happens in the space below. Then on a new line give a brief description of what your results mean and how they were obtained.
Your response (start in the next line):
0.631, 0.587, 0.603, 0.651
0.691, 0.654, 0.701
0.673, 0.649, 0.699, 0.713
0.664, 0.627, 0.689, 0.720
These results represent the data obtained from the four trials. For each trial, the pendulum was clocked at every second, fourth, sixth, and sometimes eighth, strikes of the pendulum on the bracket.
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Now measure the length of the pendulum. (For the two-pearl system the length is measured from the bottom of the 'fixed' pearl (the one glued to the top of the bracket) to the middle of the 'swinging' pearl. For the system which uses a bolt and magnet at the top instead of the pearl, you would measure from the bottom of the bolt to the center of the pearl). Using a ruler marked in centimeters, you should be able to find this length to within the nearest millimeter.
What is the length of the pendulum?
Your response (start in the next line):
6.5 cm
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If you have timed these events accurately, you will see clearly that the time from release to the second 'hit' appears to be different than the time between the second 'hit' and the fourth 'hit'.
On the average,
how much time elapses between release and the second 'hit' of the pendulum,
how much time elapses between the second and fourth 'hit' and
how much time elapses between the fourth and sixth 'hit'?
Report your results as three numbers separated by commas, e.g.,
.63, .97, .94
Your response (start in the next line):
0.665, 0.629, 0.673
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A full cycle of a free pendulum is from extreme point to equilibrium to opposite extreme point then back to equilibrium and finally back to the original extreme point (or almost to the original extreme point, since the pendulum is losing energy as it swings)..
The pearl pendulum is released from an 'extreme point' and strikes the bracket at its equilibrium point, so it doesn't get to the opposite extreme point.
It an interval consists of motion from extreme point to equilibrium, or from equilibrium to extreme point, how many intervals occur between release and the first 'hit'?
Your response (start in the next line):
One interval occurs from the release point (extreme point) to equilibrium. (1/4 of a full cycle)
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How many intervals, as the word was described above, occur between the first 'hit' and the second 'hit'? Explain how your description differs from that of the motion between release and the first 'hit'.
Your response (start in the next line):
Two intervals occur between the first hit and the second hit. The pendulum must travel from equilibrium (first hit) back to its extreme point (one interval) and then from that extreme point back to equilibrium (two intervals). This differs from the previous problem because the pendulum is starting at the extreme point and heading towards equilibrium so only one interval is needed.
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How many intervals occur between release and the second 'hit', and how does this differ from the motion between the second 'hit' and the fourth 'hit'?
Your response (start in the next line):
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How many intervals occur between the second 'hit' and the fourth 'hit', and how does this differ from a similar description of the motion between the fourth 'hit' and the sixth 'hit'?
Your response (start in the next line):
Four intervals. The same amount of time intervals occur between the second and fourth hit compared to the fourth hit and the sixth hit.
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Why would we expect that the time interval between release to 2d 'hit' should be shorter than the subsequent timed intervals (2d to 4th, 4th to 6th, etc.)?
Your response (start in the next line):
With every strike of the pendulum, it begins to decrease in speed (lose momentum). Therefore, the time interval between the release and the second hit is short than the subsequent timed intervals.
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Would we expect additional subsequent time intervals to increase, decrease or stay the same?
Your response (start in the next line):
We would expect additional subsequent time intervals continue to increase. Therefore, the time between hits would increase.
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@& This wouldn't be the case if the rhythm was constant.*@
What evidence does this experiment provide for or against the hypothesis that the length of a pendulum's swing depends only on its length, and is independent of how far it actually swings?
Your response (start in the next line):
This experiment proves that it is not just the length of the pendulum that dictates its swing. The extreme point from which is it released, the positioning of the bracket, and the accuracy of the time measured are all factors that contribute to the pendulum swing length.
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Your instructor is trying to gauge the typical time spent by students on these experiments. Please answer the following question as accurately as you can, understanding that your answer will be used only for the stated purpose and has no bearing on your grades:
Approximately how long did it take you to complete this experiment?
Your response (start in the next line):
2 hours
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&#Good responses on this lab exercise. See my notes and let me know if you have questions.
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