revisionlabwavesandpulses

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course Phy232

7-20-11 about 6:30 p.m.

Note that thedata program is in a continual state of revision and should be downloaded with every lab.Items needed for experiments:

You will need the lenses and some of the other materials in the standard kit (this is the kit you should have purchased in addition to the materials that came with your CDs). If you do not have this kit you need to order it immediately.

For your experiments on waves you will need to make chains of rubber bands and paper clips. This is a simple task but it takes a few minutes. You can save time if you do this while engaged in another task that doesn't involve your hands--e.g., while watching TV (if you're one of the lucky few who have time) or talking. Make the chain out of the thicker rubber bands and the large paper clips. You should have enough paper clips to make a chain of 25 clips and 24 rubber bands; if you're a few clips or rubber bands short that's not a problem. If you have a few extra clips it's worth the trouble to make the chain a little longer, but there's no need to exceed about 30 clips.

You will also need a transparent cylindrical container. The cylinder must be uniform over at least part of its length--a tapered container is not appropriate (the mathematics of the analysis is too complex with a tapered container). A clear 2-liter soft drink bottle is good. A regular 16- 0r 24-ounce bottle is OK, and a cylindrical jelly jar would be adequate, but you will find measurements easier with a larger container. If you have a larger transparent cylindrical container you may of course use it.

Some kits were packed with only one tea-light candle. There should have been two. If you received only one, send the instructor your mailing address. You will need two candles for some of the experiments you will do next week.

Preliminary observations

Fundamental Mode of a Rubber Band Chain

Attach one end of your clip-and-rubber-band chain to a fixed object. You may bend the end clip into a hook an hook it over a door hinge or other object, or you may place the end hook on the edge of a table or shelf and set a reasonably heavy object on it to keep it in place.

You won't want to stretch the chain out too much. Its length should never exceed its unstretched length by more than 20%. So you won't be exerting forces more than a few Newtons.

Suspend the chain by its middle, so that the two ends are hanging freely. Measure the distance from the point of suspension to each of the ends. If you add these distances you will get an approximate reference 'unstretched' length for the chain. Of course there is a little stretching due to the weight of the chain itself, but that will be fairly minimal.

When you have one end fixed, walk backward with the other end, keeping it at the same height as the fixed end, until the middle of the chain comes up off the floor. At this point the middle of the chain will probably be sagging a few feet below the ends.

• Allow the chain to sway gently back and forth, giving it an occasional gentle nudge, and observe how many times it moves back and forth in 10 seconds.

• Measure how far the middle is below the ends, and how far apart the ends are for this trial.

• Still holding the chain at the same position, give the chain a 'pluck' at that end, something like plucking a guitar string. This will send a pulse down the chain and back; the pulse will probably reflect from your hand and travel back down the chain before returning to your hand. You should be able to see three or more down-and-back pulses. Using a watch with a second hand, or something that will give you similar accuracy, take the data you will need to determine how fast the pulse is moving.

Then move backwards a little further, so that the middle of the chain is only about a foot below the ends, and repeat all measurements and timings.

• When measuring the speed of the pulse, time as many down-and-back pulses as you can reliably observe. At some tensions the chain can produce ten or more easily-counted round-trip pulses.

Finally move backwards until the chain is about 20% longer than its maximum unstretched length, and repeat once more.

In the box below give in the first comma-delimited line the results for the first trial which will include

• number of cycles in 10 seconds,

• distance in cm between the ends

• the distance of the middle below the ends and

• the speed of the pulse.

In the second and third lines give your results for the second and third trials, in the same format.

In the fourth line give the approximate unstretched length.

Starting in the fifth line give a summary of what you did and a brief explanation of what your data lines mean.

• Include also a brief report of the data you used to determine pulse speeds and how you used it to determine those speeds.

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Items needed for experiments:

For your experiments on waves you will need to make chains of rubber bands and paper clips. This is a simple task but it takes a few minutes. You can save time if you do this while engaged in another task that doesn't involve your hands--e.g., while watching TV (if you're one of the lucky few who have time) or talking. Make the chain out of the thicker rubber bands and the large paper clips. You should have enough paper clips to make a chain of 25 clips and 24 rubber bands; if you're a few clips or rubber bands short that's not a problem. Simply alternate the clips and rubber bands, starting and ending with a clip. You will end up with a chain 2 or 3 meters long. If you have a few extra clips it's worth the trouble to make the chain a little longer, but there's no need to exceed about 30 clips.

Preliminary observations

Fundamental Mode of a Rubber Band Chain

You won't want to stretch the chain out too much. Its length should never exceed its unstretched length by more than 20%. So you won't be exerting forces more than a few Newtons.

Ą Allow the chain to sway gently back and forth, giving it an occasional gentle nudge, and observe how many times it moves back and forth in 10 seconds.

Ą Measure how far the middle is below the ends, and how far apart the ends are for this trial.

Ą Still holding the chain at the same position, give the chain a 'pluck' at that end, something like plucking a guitar string. This will send a pulse down the chain and back; the pulse will probably reflect from your hand and travel back down the chain before returning to your hand. You should be able to see three or more down-and-back pulses. Using a watch with a second hand, or something that will give you similar accuracy, take the data you will need to determine how fast the pulse is moving.

Then move backwards a little further, so that the middle of the chain is only about a foot below the ends, and repeat all measurements and timings.

Ą When measuring the speed of the pulse, time as many down-and-back pulses as you can reliably observe. At some tensions the chain can produce ten or more easily-counted round-trip pulses.

Finally move backwards until the chain is about 20% longer than its maximum unstretched length, and repeat once more.

In the first comma-delimited line below, give the results for the first trial which will include

Ą number of cycles in 10 seconds,

Ą distance in cm between the ends

Ą the distance of the middle below the ends and

Ą the speed of the pulse.

In the second and third lines give your results for the second and third trials, in the same format.

In the fourth line give the approximate unstretched length.

Starting in the fifth line give a summary of what you did and a brief explanation of what your data lines mean.

Ą Include also a brief report of the data you used to determine pulse speeds and how you used it to determine those speeds.

Your answer (start in the next line):

7, 184, 58.4, .7cycle/sec

9, 232, 10.4, .9cycle/sec

19, 290, 0, 1.9cycle/sec

242.4

1st trial I gently nudged the chain and counted how many times the wave completed 1cycle in 10 sec, 2nd trial I pulled the chain tighter by moving back more and gave the chain a good pluck and counted as before, and the 3rd trial I moved further back and give the chain a good pluck again and did as before. The data for each line is in this order # of cycles in 10sec, length of the chain end to end in cm, sag in the middle in cm, and the speed of the pulse.

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Now give the following:

Ą In the first line: first stretch cycles in 10 seconds, dist between ends, dist of middle below ends, speed of pulse:

Ą In the second line: second stretch cycles in 10 seconds, dist between ends, dist of middle below ends, speed of pulse:

Ą In the third line: third stretch cycles in 10 seconds, dist between ends, dist of middle below ends, speed of pulse:

Ą In the fourth line: approximate unstretched length:

Ą In the fifth line: how you determined pulse speeds from data:

Ą In the sixth line: your brief discussion/description/explanation:

Your answer (start in the next line):

7, 184, 58.4, .7cycle/sec

9, 232, 10.4, .9cycle/sec

19, 290, 0, 1.9cycle/sec

242.4

Speed calculated by #cycles/10sec, example 7cycles/10sec= .7cycle/sec

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@& Speed is distance / time interval.

How much distance is covered in a cycle?

How does this change your answers for the speed?*@

&&&&1st trial 18.4cm/sec, 2nd trial 23.2cm/sec, 3rd trial 29.0cm/sec&&&&

@& You haven't answered the questions

How much distance is covered in a cycle?
How does this change your answers for the speed?

You report the length of the chain as 242 cm, and in at least one trial the pulse completes more than a complete round trip in one second. This gives you speeds that are at least in the hundreds of cm / sec.*@

Focusing of light by a cylindrical lens

For this activity you will use a cylindrical container filled with water (most soft drink bottles have a cylindrical section and work fine if the label is peeled off; most drinking glasses are tapered and will not work well), a concentrated single source of light (e.g., a single bulb, a candle, even a flashlight), and a hardbacked book (your physics text will work fine). The cylinder should be on a flat surface (for example a tabletop or a counter), and the source of light should be at least 5 feet and preferably 10 feet away from the cylinder and vertically at about the same level as the cylinder. The book will be placed on the level surface 'behind' the cylinder, so that light passes through the source before reaching the book. The book should be standing on its end, facing the cylinder and the light source. All other light sources should be dimmed or turned off.

The setup is similar to the one shown in the picture below. The candle is the source, and your book will take the place of the block of marble standing on its edge near the right boundary of the picture. The book can simply stand on the tabletop. However the candle or other light source should be raised to about the level of the center of the container, and should be further from the bottle.

If you move the book closer to or further from the cylinder, you will see that the light passing through the cylinder is focused to form a vertical band, which gets wider or narrower depending on how you move the book. At a certain distance from the book, this vertical band is narrower and sharper than at any other distance.

Position the book at this distance, so that the light forms the narrowest, sharpest possible band. Measure the distance from the light source to the cylinder, and the distance from the cylinder to the book. Also measure the diameter of the cylinder.

Report your measurements and describe in detail how they were made:

Your answer (start in the next line):

309.4cm is the distance from light source to the cylinder, distance from cylinder to the book is 5.2cm, and the diameter of the cylinder is approximately 10cm. Measurements made by measuring the distance of the flashlight from the cylinder, then moving the book closer and further away from the cylinder to see where the narrowest band of light was.

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Now move the light source four times as close and repeat your measurements. Give your results in the box below:

Your answer (start in the next line):

232.4cm from flashlight to cylinder, 4.6cm from cylinder to book

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Continue repeating (moving 4 times closer each time) until the light source is so close to the bottle that you can't form a sharp vertical line. This won't require many steps--three or four at most will bring the light almost to the surface of the cylinder.

Report your results below:

Your answer (start in the next line):

155.4cm from cylinder to light, 4.2cm from cylinder to book

78.4cm from cylinder to light, 4.8cm from cylinder to book

1.4cm from cylinder to light, cannot form a sharp beam.

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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:

Ą Approximately how long did it take you to complete this experiment?

Your answer (start in the next line):

2 hours

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@& Good, but you need to modify your results for the speed of the pulse in the rubber band chain. This probably won't take you long at all.

&#Please see my notes and, unless my notes indicate that revision is optional, submit a copy of this document with revisions and/or questions, and mark your insertions with &&&& (please mark each insertion at the beginning and at the end).

Be sure to include the entire document, including my notes.

If my notes indicate that revision is optional, use your own judgement as to whether a revision will benefit you.

@& Your revised answers aren't consistent with your reported data. You don't say how you got those answers. You're doing something wrong but I can't tell what it is. It will be very easy to correct, if necessary, provided you explain the details of your reasoning.

Can you submit a copy of your data and your reasoning on the speed? It shouldn't take more than a couple of lines. *@