String wave primer

[frankglennjacobs@gmail.com]

My advisors on Science Forum are right: Instead of just speculating, I should DO some physics thing and record what happens.

 

I didn't have enough rope to to the rope wave thing, but, BEHOLD! here is this stick with builder's nylon twine wound on it. I measured off eleven meters of it, made loops ten meters apart, hung one end on a fence post and held the other end in my hand, hanging nearly to the ground.

 

I even wrote a "null hypothesis": This lightweight stuff isn't going to make waves.

 

It didn't. Not just loosly hanging like that. All it made was a parabola.

 

So I took a step backward. The string made almost a straight line. But it made a wave. Not having a way to time it, I estimated the wave went the ten meters in less than a second. Not only that, it reflected back my my hand and then back to the fence. And back. And back. And back. The wave was plainly visible TWELVE trips -- well, anyway, six round-trips.

 

Hot DOG! I am doing science! Seventy-five years old and doing science fit for a six-year-old.

 

Oh. 7,000 feet above sea level. Eighty degrees Farenheit. Zero wind. Waves going east and west. Clear blue sky. No local traffic. Moderately-curious horse looking at the whole business.

 

I repeated the exercise several times with the same results.

Edited June 17, 2017 by frankglennjacobs@gmail.com

[KipIngram]

Wait - are you trying to claim that a string under tension doesn't propagate waves? If so, you're wrong - your null results reflect a flawed experimental setup.

 

All stringed musical instruments work on this principle.

[imatfaal]

My advisors on Science Forum are right: Instead of just speculating, I should DO some physics thing and record what happens...

Actual empiricism is always good. But you must be careful that your method and observations are able to disprove your null hypothesis

 

 

...I didn't have enough rope to to the rope wave thing, but, BEHOLD! here is this stick with builder's nylon twine wound on it. I measured off eleven meters of it, made loops ten meters apart, hung one end on a fence post and held the other end in my hand, hanging nearly to the ground.

 

I even wrote a "null hypothesis": This lightweight stuff isn't going to make waves.

 

It didn't. Not just loosly hanging like that. All it made was a parabola.

....

Behold what?

 

What was the method used to test the null - did you just hold a piece of string?

 

And the shape it made was a catenary (or close to) rather than a parabola. See a scientist would either have looked that up (and given references) or experimented themselves - what they would not have done was give a bald assertion because that could have been culpably wrong and it is.

 

[latex]y = a \cosh{\frac{x}{a}}[/latex]

 

rather than [latex] y=ax^2+bx+c [/latex]

 

https://en.wikipedia.org/wiki/Catenary

 

So I took a step backward. The string made almost a straight line. But it made a wave. Not having a way to time it, I estimated the wave went the ten meters in less than a second. Not only that, it reflected back my my hand and then back to the fence. And back. And back. And back. The wave was plainly visible TWELVE trips -- well, anyway, six round-trips.

 

Hot DOG! I am doing science! Seventy-five years old and doing science fit for a six-year-old.

 

Oh. 7,000 feet above sea level. Eighty degrees Farenheit. Zero wind. Waves going east and west. Clear blue sky. No local traffic. Moderately-curious horse looking at the whole business.

 

I repeated the exercise several times with the same results.

So do you consider that your results show that the null hypothesis is not true?

[Strange]

Wait - are you trying to claim that a string under tension doesn't propagate waves? If so, you're wrong - your null results reflect a flawed experimental setup.

 

 

I took it to mean the reverse: there were waves only when it was under tension. I guess a loose string is too damped to propagate.

[frankglennjacobs@gmail.com]

A rope will make waves as long as the tension keeps it off of the ground.

 

My null hypothesis indicated that I believed such a lightweight string would not make waves because the air resistance would damp them out.

 

It also made a second wave -- that was heard but not seen.

 

I see that this is not exactly "pioneer science".

 

Now what I need is a way to measure the tension and a way to measure the time.

[Mordred]

A rope will make waves as long as the tension keeps it off of the ground.

 

 

Umm no definition in physics.... How can you possibly model that sentence. Nothing personal but one of the avenues to understand physics besides mathematics,is also terminology and grammar usage. Edited June 18, 2017 by Mordred

[imatfaal]

A rope will make waves as long as the tension keeps it off of the ground.

 

My null hypothesis indicated that I believed such a lightweight string would not make waves because the air resistance would damp them out.

 

It also made a second wave -- that was heard but not seen.

 

I see that this is not exactly "pioneer science".

 

Now what I need is a way to measure the tension and a way to measure the time.

 

Measure tension - use an A-frame. String attached to apex. Legs angled towards fixed fencepost Weight attached to cross-bar. Angle of frame WRTo ground varies tension in string. Won't give you absolute value in SI units but a bit of trig will give you very nice relative values for different angles

Measure time - you got a watch with a second hand? Or much better a smartphone with video. Download Tracker and use serious tools to get timings and shapes of waves - its free and pretty easy to use

[frankglennjacobs@gmail.com]

I looked around for an A-frame. I have several as they make up the frame of my barn, but I can no longer get up to the top of them to connect anything up there. I would like to have put a pulley up there to be able to hoist, um, whatever, thirty-five feet off the ground, out of the sun and wind.

 

I got one of those cheap "spring balance" scales and repeated the horizontal exercise.

 

Contrary to what I had seen (thought I saw?) the other day, the string made waves when under barely enough tension to clear the ground. Not neat little horizontal waves, but only big circular waves when I moved my hand like a Hollywood movie steam-train brakeman swinging a red lantern.

 

I was not able to make the same kind of waves as before. I tried several tensions.

 

The only difference was the presence of the spring-scale. Maybe the spring undid the horizontal waves.

 

Maybe my mind-set was wrong and I waved the string wrong. It sort of buzzed like a really low-pitched cello.

 

Maybe my mind-set was wrong and I saw things that weren't there or missed things that were.

 

I took a nap to let my mind get clear. However, the sky got clear dark and I took another nap and wrote this up.

[DrP]

You have to put some energy into it to get a wave to propagate along a rope... it will loose energy all the way as the wave moves along the rope 'lifting' the rope up as it travels. Tie one end to a post and shake the other end. With the shaking, depending on how hard and often, you can control the amplitude and frequency of the wave as it propagates along the rope. When you shake it, you will see the wave you made move along the rope.

 

You can even see a wave propagating along a totally limp rope laid straight out along the ground. 'Whip' one end up and down sharply and the up/down motion will travel as a wave along the rope until it runs out of energy or until it reaches the end of the rope.

[frankglennjacobs@gmail.com]

Today I tried it all out again with the string. (This is about one mm diameter nylon twine the builders use to lay out trenches and brick lines.)

 

When I put even one KG of force on the string, it stopped making visible waves. However, it made a wave that I could not see. I could feel it coming back to my hand in about half a second.

 

I did not get visible waves as long as the string was pulled nearly straight. It had to have visible slack to make visible waves.

 

My null hypothesis is that my mind has gone null and that I am not seeing what I think i should see.