Intro to Physically Reasonable Waves on a String

[Redbelly98]

Introduction
Physics teachers who are either writing physics questions that deal with waves on a string or setting up equipment for a class lab or demo of standing waves on a string might find the following analysis useful. When writing questions for physics tests or homework, it is preferable to use physically plausible values for any quantities (Ref. 1). With that in mind, let us explore what physically limits quantities such as wave speed or string tension in a typical standing-wave setup for a school physics lab.

The above figure shows a typical standing-wave setup. An oscillator at one end of a string vibrates vertically to produce waves along the string, while the weight hanging at the other end holds the string taut. A pulley confines the waves to the horizontal section of the string while still allowing the force exerted by the weight to hold the string taught.
The...

Continue reading...

 

[Redbelly98]

I'm particularly interested in hearing from physics teachers or students who have used these setups, especially if your setup was outside what I'm calling the "physically reasonable zone".

 

[sophiecentaur]

if your setup was outside what I'm calling the "physically reasonable zone".

I'm not sure what you mean by that expression, exactly but I have used an alternative method which might interest you. The method involves electromagnetic force. A nice beefy horseshoe magnet is placed with a stretched steel wire held between the poles. One end is fixed and the other is tensioned with a variable mass. A fairly powerful signal generator is connected across the ends of the wire. Not directly on the wire but to the fixing and pulley so that the excitation is 'clean' and can be applied at any point along the wire. I prefer this method because, unlike when the end of the string is moved, the precise wavelength can be measured because true Nodes can be seen at each end.
Good magnets are easily obtainable these days - much better ones than what I had available a few years ago and a standard School Oscillator will give you frequencies from a few Hz to a few tens of Hz.
Give it a go.

 

[Redbelly98]

Oh, cool setup!

I'm not sure what you mean by that expression ...

I was referring to the two diagrams toward the of the article -- to see them, you'd need to click the "Continue reading" link. (Not sure if you didn't see the diagrams, or did see them and something still isn't clear about them.)

 

[sophiecentaur]

(Not sure if you didn't see the diagrams, or did see them and something still isn't clear about them

Ah yes - I see where you're coming from. now. Choosing suitable ranges of variables to present to students for practical exercises can be approached in two ways. you either tell them to look between specified limits or you tell them to find those limits. Personally, I always found it difficult to find students with enough confidence to do the latter. They always wanted guidance and we seldom had time to do any more than the basic experiment, followed by the customary graph and analysis. I sometimes wonder if it had anything to do with the religious basis on which the School was run - but that could be just me offloading some of my own guilt.
Afair, there were only one or two opportunities for extended investigations and there would only be the occasional student who would actually risk writing their own agenda. When that occurs, of course, it's great.
Spotting where the experiment breaks down - where the graph leaves the straight line, for instance is good for qualitative comments.

Earlier in life, as a Research Engineer, I was always coming across this sort of thing but I think experiments would have been run, using ranges of variables that were realistic, found during the experiment on an iterative basis (seat of pants, you could say).