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course Phy 121
8:20 June 4
introductory pendulum experiment#$&*
Phy 121
Your 'introductory pendulum experiment' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
** Introductory Pendulum Experiment_labelMessages **
** **
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In this experiment you will make a simple pendulum and observe how its frequency of oscillation varies with its length.
The goals of this experiment include the following:
Practice sketching graphs.
Practice interpreting graphs.
Obtain a concrete example which will be useful in understanding the essential concept of rate of change.
Enhance your ability to make accurate observations.
Quantify limits on the accuracy of data and the effect of these limits.
Obtain information necessary to use a pendulum as an accurate timing device.
To make the pendulum:
Tie a light string or thread, about as long as you are tall, around a relatively small dense object.
In the absence of anything more convenient you could use a couple of CDs or DVDs with a string or thread tied through the middle.
A ring might also be a good choice (but avoid using a ring with a lot of value).
A smaller and denser object would be preferable, but don't take a lot of time trying to locate the smallest densest object you can find. Just about anything will do if it is smaller than an average fist and dense enough to sink if it is placed in water (you won't actually be placing the object in water).
The string or thread should be about 4 feet long.
If you have a ruler or a measuring tape marked in centimeters, you may use it. If you don't, open and print out the file at the link ruler, which can be used to obtain sufficiently accurate measurements.
You will count and time oscillations of a series of simple pendulum, of 9 different lengths.
Determine the lengths of the pendulums to be used in your experiment:
Take your height in inches, subtract 30 and divide your result by 5. Round your result off to the next whole number. This will be the length in centimeters of your first pendulum. (For example if you are 91 inches tall (unlikely but not impossible), you would subtract 30 to get 61, then divide 61 by 5 to get 12.2. This rounds off to 12, so your pendulum would be 12 cm long).
Double the length of your first pendulum. This is the length of your third pendulum.
Double the length of your third pendulum. This is the length of your fifth pendulum.
Double the length of your fifth pendulum. This is the length of your seventh pendulum.
Double the length of your seventh pendulum. This is the length of your ninth pendulum.
Write down the numbers 1 through 9 in the first column of a table, and the lengths you have obtained in the second column, each length opposite the appropriate number. For example if your first pendulum was 12 cm long your table would look something like the following:
number length
1 12
2
3 24
4
5 48
6
7 96
8
9 192
Note that you aren't asked here to enter your table.
Sketch a rough graph of your lengths
You are going to make a graph of this information.
Sketch the graph by hand (perhaps using the template below). Don't use the computer to construct your graphs in this experiment.
If you want to print and use the template below you may do so, or you may make a rough sketch something like the figure below. Don't bother to use a ruler and make a meticulous graph. Any reasonable freehand sketch is fine (or again, if you wish, you can print the figure below and use it).
Now make a graph of pendulum length vs. number, based on your table. This graph will have length on the vertical axis and the pendulum number on the horizontal. The graph will look something like the one below, which however is based on the unlikely 12 cm initial length for the 91-inch-tall student.
Sketch a smooth curve through the points on your graph. This will give you a graph much like the one below:
Estimate the pendulum lengths which would correspond to numbers 2, 4, 6, and 8.
Complete the table by filling in the lengths corresponding to pendulums number 2, 4, 6 and 8, as estimated from your graph. Try to make reasonable estimates, but don't take a lot of time to make your estimates exact. If you are 91 inches tall your table might be as indicated below:
number length
1 12
2 15
3 24
4 35
5 48
6 70
7 96
8 140
9 192
Time and count oscillations for the nine different pendulums
Hold the string so that the length from the point at which you are holding it to the center of the object is equal to the first distance on your table (e.g., for the table given above that distance would be 12 cm).
Start the pendulum oscillating, but don't make it swing too far--keep the distance from one end of the swing to the other less than half the length of the pendulum. To keep it swinging you might need to use the fingers in which you are holding it to slightly nudge the pendulum.
Using a clock with a second hand, determine how many times this pendulum oscillates in 60 seconds. A complete oscillation, or a complete cycle, is from one extreme point to the other and back. If you count every time your pendulum changes direction, you are counting half-cycles (it's fine to count half-cycles; if you do just be sure to divide by 2 to get the number of complete oscillations). Repeat your count at least a few times, and continue until you are sure you know to the nearest whole cycle (or if you prefer to the nearest half-cycle) how many times it oscillates back and forth in a minute. It shouldn't take more than a few trials to get in the habit of counting accurately, but if you don't count carefully it might. A surprising number of students have difficulty making an accurate 60-second count, and some need the practice.
Repeat this for each of the nine lengths on your table.
Construct a table of the number of oscillations observed vs. the length of the pendulum. For the example given here, corresponding to a person 91 inches tall, the table might look something like the following (the numbers in this example shouldn't be regarded as particularly accurate or useful; your counts should be more accurate than these):
length number of cycles
12 85
15 75
24 60
35 50
48 40
70 35
96 30
140 25
192 20
Graph your data
You will graph the data in your table.
You may if you wish use the template below to help you construct your graph; you may if you wish print out the figure, or simply make a hand sketch--again be reasonably accurate but don't take the time to be be overly meticulous.
A graph of the sample data for the very tall student would look like the following:
Figure out the time required per oscillation at each length
You know how many times each pendulum oscillated in 60 seconds. From this information you can figure out for each length how many seconds, and/or what fraction of a second, was required for a single oscillation. You should be able to figure out how to do this. Make a table showing the number of seconds required per oscillation vs. the length of the pendulum in cm. The figure below corresponds to the example of the 91-inch-tall individual, and these results are not to be regarded as particularly accurate.
length time for one cycle
12 0.7
15 0.8
24 1
35 1.2
48 1.5
70 1.7
96 2
140 2.4
192 3
Construct a graph of the time required for a single oscillation vs. the length of the pendulum. You may use the graph below as a template:
For the present example the graph might look like this:
Sketch a smooth curve to represent the data. A good smooth curve for the sample data represented above might look like this:
The curve should come as close as possible, on the average, to the data points, but it should not 'wobble around' in an attempt to actually go through any of the data points. The figure below represents a smooth curve that does wobble around a lot, and this isn't what you want here:
After the 'Your answer' prompt about half a page below you will report your data, using the precise format described below. All reported data will be collected and reported back to the rest of the class (your data will be reported anonymously--only I will know whose data is which), and if anyone's data is not in the prescribed format, it might be necessary to ask everyone to report their data again.
Be sure to save your information as you type it into the copied document. Keep your copies; if the data is required later you will then be easily able to access it.
After the prompt on the first line below the 'Your Answer' prompt, report your vccs email address, the length of the shortest pendulum in centimeters, the number of cycles counted for this pendulum in 60 seconds, and the time required for one complete cycle. Put a comma between each pair of entries. So for the example of the 91-inch-tall student, the first line would read
abc123@email.vccs.edu, 12, 85, .7
Each subsequent line will appear in the same format. So the next two lines for this example student would be
abc123@email.vccs.edu, 15, 75, .8
abc123@email.vccs.edu, 24, 60, 1
It should take you only a couple of minutes to enter this information. You should as usual use copy-and-paste to insert your email address (this will save you time and will ensure that you have given the correct address).
Lines have been provided for up to 8 lengths; however if you followed the instructions you should have observed pendulums of only 6 or 7 different lengths. Extra lines can be left blank.
Your answer (start in the next line):
length of shortest pendulum, number of cycles in 60 sec, time for each cycle:
8 cm, 68 cycles, .88
length of next-shortest pendulum, number of cycles in 60 sec, time for each cycle:
14 cm, 57 cycles, 1.05
length of next-shortest pendulum, number of cycles in 60 sec, time for each cycle:
16 cm, 54 cycles, 1.11
length of next-shortest pendulum, number of cycles in 60 sec, time for each cycle:
26 cm, 46 cycles, 1.30
length of next-shortest pendulum, number of cycles in 60 sec, time for each cycle:
32 cm, 43 cycles, 1.40
length of next-shortest pendulum, number of cycles in 60 sec, time for each cycle:
46 cm, 38 cycles, 1.58
length of next-shortest pendulum, number of cycles in 60 sec, time for each cycle:
64 cm, 32 cycles, 1.88
length of next-shortest pendulum, number of cycles in 60 sec, time for each cycle:
92 cm, 29 cycles, 2.07
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If a pendulum completes 29 cycles in a minute, then since there are 60 seconds in a minute, the time required for a cycle is more than a second.
If the time for each cycle was .5 second, then 29 cycles would take only 29 * .5 second = 14.5 seconds, not a minute.
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### I found the equation to find the time for each cycle and did 60/number of cycles. I see my mistake, I had previously done number of cycles/60.
your brief discussion/description/explanation:
The longer the pendulum, you start to see it decreasing.
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1. According to your graphs, complete the following tables
length in cm number of cycles time for one cycle
10 64 .94
30 44 1.36
50 35 1.71
70 31 1.94
90 30 2
110 28 2.14
130 22 2.73
Enter the numbers from your table in the space below with one line for each length. Each line should contain the length, number of cycles and time for one cycle, separated by tabs.
Your answer (start in the next line):
#$&* length, count, period for given lengths
length in cm number of cycles time for one cycle
160 10 6
60 30 2
20 50 1.2
8 70 .86
3 90 .67
Enter the numbers from your table in the space below with one line for each length. Each line should contain the length, number of cycles and time for one cycle, separated by tabs.
Your answer (start in the next line):
#$&* length, count, period for given counts
length in cm number of cycles time for one cycle
App 62 30 0.5
App 16 54 0.9
App 4 78 1.3
App 3 102 1.7
App .9 126 2.1
App. 5 150 2.5
Enter the numbers from your table in the space below with one line for each length. Each line should contain the length, number of cycles and time for one cycle, separated by tabs.
Your answer (start in the next line):
#$&* length, count, period for given periods
2. Is the graph of # of cycles vs. length in cm constant, increasing or decreasing? Is it doing so at an increasing, constant or decreasing rate?
Decreasing at a decreasing rate. Graphed length on the y axis and number of cycles on the x
On this and on all questions, insert your answer after the 'Answer:' prompt, and include a brief explanation of how you arrived at your answer.
Your answer (start in the next line):
#$&* graph of count vs length
3. Is the graph of time required for one cycle vs. length in cm constant, increasing or decreasing? Is it doing so at an increasing, constant or decreasing rate?
Where the time required is all 60, it would be constant, decreasing rate
Your answer (start in the next line):
#$&* graph of period vs length
4. How much difference is there between your first two lengths, and how much difference between the number of cycles counted in 60 seconds?
Your answer (start in the next line):
First length was 8 and the second length was 14, so the difference was 6. The first had 68 cycles and the second had 57= 11. A slight difference
#$&* diff between lengths, between counts
5. How much difference is there between your first two lengths, and how much difference between the corresponding times required to complete a cycle?
Your answer (start in the next line):
First length was 8 and the second length was 14, so the difference was 6. The first had .88 and the other 1.05= difference of .17.
#$&* diff between lengths, periods
6. How much difference is there between your last two lengths, and how much difference between the number of cycles counted in 60 seconds?
Your answer (start in the next line):
The last two lengths were 92 and 128. The difference is 36. The last two counts were 29 and 25= 4.
#$&* diff between last two lengths, counts
7. How much difference is there between your last two lengths, and how much difference between the corresponding times required to complete a cycle?
Your answer (start in the next line):
The last two lengths were 92 and 128. The difference is 36. The last two periods were 1.88 and 2.07= difference .19.
#$&* diff between last two lengths, periods
8. Is your graph of number of cycles counted vs. length in cm steeper, on the average, between the first two lengths or between the last two lengths?
Your answer (start in the next line):
#$&* count vs. length steeper between 1st two or last two pts
9. Is your graph of time required to complete a cycle vs. length in cm steeper, on the average, between the first two lengths or between the last two lengths?
Your answer (start in the next line):
It appears to be steeper between the last two lengths
#$&* period vs. length steeper between 1st two or last two pts
10. The curve you sketched for your graph of (time required to complete a cycle) vs. (length) cannot possibly pass through the center of each of your points.
What is the greatest vertical distance between a point of your graph and the curve?
What do you think is the least vertical distance?
You will answer these questions at the 'your answer' prompt a little ways below.
(For example, in the figure below a curve has been constructed based on three data points. The first and third data point lie slightly above the curve, the second point slightly below. The second point is probably the one which lies furthest from the curve, at a distance of approximately .03 vertical units below. This distance is roughly estimated based on the scale of the graph. The first point is perhaps .01 vertical units above the curve, and the third is perhaps .02 units above.)
Your answer (start in the next line):
Distance between last two points (sec per oscillation) 2.1-1.9= .2 and (length) 92-64= 28.
#$&* greatest, least vert dist between datapt and curve
After the 'Your Answer' prompt below, insert your answers to the following :
Describe how you constructed your pendulum and out of what (what you used for the mass, its approximate dimensions, what it is made of, what sort of string or thread you used--be as specific as possible).
I used a string and a dvd to construct my pendulum. I tied the string through the middle of the dvd and cut the string according to my height. Circular in shape and very light.
Describe its motion, including an estimate (you don't have to measure this, just give a ballpark estimate) of how far it swung from side to side and how this distance varied over the time you counted.
Estimating, it probably swung a little over a foot or so
Describe what you mean by a 'cycle'. Different people might mean different things, but there are only a couple of reasonable meanings. As long as you describe what you mean we will all understand what you measured.
I counted a cycle from the beginning point, swinging to the other side and then coming back to the start as one whole cycle.
'Frequency' means the number of cycles in a unit of time. Your counts are frequencies, in cycles/minute. 'Period' means time required for a cycle. Explain how you used your observed frequencies to obtain the periods of the nine pendulums in this experiment.
Your answer (start in the next line):
Frequencies is the number of cycles in a unit of time. For this experiment, we used 60 seconds. To find the periods, I took 60/by the number of cycles.
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Close, but not quite. You need to rethink this procedure, per my previous note.
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#$&* explanations with std terminology
*#&!*#&!
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You have good data, and for the most part your analysis is good, but you made a fundamental error in calculating the time required for a cycle.
This affects a number of your subsequent results.
Please revise your document and resubmit. "
Self-critique (if necessary):
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Self-critique rating:
________________________________________
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Very good. I knew you would recognize your error.
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