PHY 201
Your 'pearl pendulum' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
** #$&* Your general comment, if any: **
June 19 around 11:00am
** #$&* Your description of the rhythm of the pendulum when tilted 'back' **
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 like this:
When suspended from the pendulum bracket by a thread the system might look something like this. 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.
I think that the pearl's sound has an increase or gets faster as the pearl starts to slow down the distance from the pendulum of the swings.
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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.
I think that the sounds remain at a steady rhythm, because of the incline of the metal bracket on the bottom.
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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..
When I placed the metal bracket on the flat surface, the pearl was touching the bracket, so I put a quarter under neath the bracket and that was too high, bacause the pearl was not touching bracket at all. So, I used a dime and that was just enough for the pearl to just touch the bracket ever so slightly. When releasing the pendulum, I heard a constant rhythm. The pendulum hit the metal bracket approximately 9 times until it came to rest.
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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.
When I placed the bracket on the book parallel to the sides, I released the pendulum and found that the sound was got faster as it the pearl came to a complete stop.
Then, I moved the bracket 45 degrees to the right, I found that the sound got faster compared to the initial position.
Next, I moved the bracket another 45 degrees, which is 90 degrees from the initial position. This time the progression of the pendulum got even faster than the other two position's, so the progression of the sound is increasing.
When I moved the bracket another 45 degrees, or 135 degrees from the initial posistion, I noticed this time that the sound progressions was declining ever so slightly.
Next, I then moved the bracket another 45 degrees, or 180 degrees from the initial position. The sound progression was declining more than the recent position.
Then, I moved the bracket another 45 degrees, or 225 degrees from the initial position and released the pendulum. This time found that the sound progression was going down, or getting slower.
When I moved the bracker another 45 degrees, or 270 degrees, I noticed that the sound had gotten slower from the last poistion.
Then, I rotated the bracket a last 45 degrees to my starting point and noticed this time that the pendulum had increased fromt the recent position.
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Describe how you would orient the bracket to obtain the most regular 'beat' of the pendulum.
I think to get the most regular beat of the pendulum, I would place it at 270 degrees, or when the bracket is facing 9 o'clock, because the incline makes a huge differnce on the progression of the sounds.
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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.
.391
.375
.328
.313
.297
.312
.336
.359
First starting at the position I picked that was the most regular beat, I went around 45 degrees at a time and measured the release of the pearl until the second hit of the pearl on the bracket, and that was my timed interval.
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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, and 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.
.6089, .5770, .4680, .5460
.5610, .4990, .4520, .4840
First, I place the bracket at the position where I picked the most regular beat is at, and went 90 degrees each time instead of 45 degrees, because it stated that it only need 4 trails instead of eight. The top numbers are the time intervals from the second to the fourth hit of the pendulum, and the bottom row of numbers are the time intervals from the fourth to the sixth hit of the pendulum.
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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?
8.6 cm’s is the length of my pendulum.
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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
.339, .545, .500
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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'?
If an interval consists of a motion from extreme point to equilibrium, then there is only 1 interval from the release to the first hit of the pendulum.
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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'.
The amount of intervals between the first hit and second hit is one interval. This time though the pendulum is going from equilibrium to a extreme point, which is the opposite of the release and the first hit.
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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'?
There are 2 intervals between the release of the pendulum and the second hit. The motion of the first and second hit has greater swings from the hit, because of the distance at which I released the pendulum at, and every time the pearl hits the bracket it is going to get weaker.
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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'?
There are 2 intervals between the second and the fourth hit. The motion of the pendulum is slowing down, or taking shorter swings after the hits. The pendulum is slowing down even more every time the pearl hits the bracket.
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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.)?
When you release the pendulum, it is going to have a greater distance to hit the bracket, which is creating greater speed when it hits the bracket. The first hit is going to be greater, which is going to send the pendulum up further than any other timed interval.
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Would we expect additional subsequent time intervals to increase, decrease or stay the same?
I think we would expect more time intervals to decrease, because every time the pendulum is hitting the bracket it is slowing down, which takes less and less time for the pendulum to travel.
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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?
The length of a pendulum’s swing depends very much on the length of the pendulum itself, because the greater distance it is from the bracket the more distance it is going to travel after it hits the bracket.
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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?
2 hours
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Good answers on most questions, and you have good data. However be sure you check the linked document and note your discrepancies. No need to submit a revision unless you have questions.
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Expanded Commentary
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