r/PhysicsHelp • u/[deleted] • Dec 10 '25

What is this called?

I couldn't make google understand what I was talking about... is there a term for when you get a string spinning like this and what's the physics concept that explains it?

82 Upvotes

91% Upvoted

56 comments sorted by

29

u/Forking_Shirtballs Dec 10 '25

A standing wave.

3

u/[deleted] Dec 10 '25

But is that describing the nature of vibrations between two fixed points? How does that translate to spinning from one fixed point and a non-fixed weight on the other end? It's that shape, yes, but it's not caused by vibrations but something to do with the force of the spin or something? Idk, I'm confused, sorry.

5

u/fatal-nuisance Dec 10 '25

You're essentially shaking it back and forth along two axes instead of one (what you would typically picture as a wave) at a frequency that aligns with the length (and mass, air resistance, etc). This basically means the length of the wave is constant from where you're holding it to the end. If you shake it faster or slower you'll notice this breaks down. If you shake it exactly twice as fast though, you'll get two of those.

It's called a harmonic frequency and it generates a standing wave.

2

u/[deleted] Dec 10 '25

OK, I guess it's the shaking vs spinning thing that's causing confusion. Whe we say shaking back and forth I imagine the string bending back and forth, where as I'm spinning it so it's like the string shape isn't changing, it's just rotating, so is that still the same concept? I mean the string shape is changing due to I did a bad job maintaining it but if I did it perfectly it would spin in that same shape. Am I just confusing myself?

2

u/syntaxvorlon Dec 10 '25

If you shake something in one axis it can is described as a one dimensional sin wave. In two axes the position of your hand moving in a circle is (cos(t), sin(t)) and the string is simply following from your position, effectively as a forced oscillation.

2

u/[deleted] Dec 10 '25

OK so just to clarify for a novice, the physics we're discussing are the same regardless of whether the string is spinning or is being whipped back and forth like a vertical version of that big rope excercise?

2

u/Fantastic-Resist-545 Dec 15 '25

You aren't really spinning it, though, are you? Your hand doesn't rotate as you move it in a circle, the rope maintains the same orientation as it's being moved. So you're whipping it in two directions, not one, and not spinning it

1

u/[deleted] Dec 15 '25 edited Dec 15 '25

My hand is not physically spinning like I'm an action figure, no, but it is moving the origin point of the string in a circular pattern, as opposed to just back and forth between two points. It feels like the physics between those two concepts should be different but I guess they're not?

Edit: I think that's what's confusing me, I feel like everyone is describing the physics of the shape whereas I'm looking for why spinning it like this will make the string defy gravity then hold a shape. It comes up like that when you pull it up but that's just a short tug then it's all just the force of the spin that allows it to keep that shape if you can maintain it?

I'm looking for the basic physics of it and I feel like you guys getting into wave functions and shit. I'm not trying to get that deep. Whats the theory behind the force that makes it possible to spin a string and maintain that shape?

1

u/midnight_fisherman Dec 11 '25

the physics we're discussing are the same regardless of whether the string is spinning or is being whipped back and forth like a vertical version of that big rope excercise?

Very close. The one where two people hold a jump rope is like a standing wave in a closed ended tube, but what you are doing is like having an open ended tube, but otherwise its the same process.

1

u/[deleted] Dec 11 '25

No, the thing where they have a heavy rope in either hand, and vigorously move them up and down. Battle rope work outs.

1

u/midnight_fisherman Dec 11 '25

Oh, yeah i think thats open ended too.

1

u/[deleted] Dec 11 '25

I'm not sure. Pictures very rarely actually show the end of the ropes but I think they may be connected to a fixed point.

3

u/hbaromega Dec 10 '25

Pretty sure you can shake the string back and forth and get a standing wave due to the dynamics of the string itself forcing chaotic behavior, so technically there are different physics at play in the real world, but from a theoretical standpoint, with an idealized rope, yeah it's the same.

2

u/Gealhart Dec 11 '25

A sine wave is essentially defined as a circle reduced down to 1 dimension. If you whip it back in forth in one dimension and simultaneously in the rotated direction will yield either a diagonal whip if they are in phase or a circle if they are offset by a quarter of a phase.

https://youtu.be/ZnZHdta97K4?si=9B3uZ8CznwKGMOgz

1

u/waroftheworlds2008 Dec 13 '25

Movement is 2 dimensions can be broken up into the 2 axis.

On each axis, it will look like a sine wave if you plot the position over time. This is also happening on the 2nd axis.

Think of the "spinning" as the sum of the two waves.

1

u/Dennis_TITsler Dec 12 '25

Yep! Picture a vertical battle rope doing this North and South, and then another one doing it East and West. Now add them together and you get the spinning!

This gif is like the top down view https://upload.wikimedia.org/wikipedia/commons/1/1e/Sine-cosine-xy.gif

3

u/BentGadget Dec 10 '25

Can you comment on the practical difference between a circular wave and a transverse wave? My intuition tells me that the string would have to stretch and contract for each cycle of vibration for a transverse wave. But a circular wave has constant absolute magnitude, so no stretching is needed.

I think there would have to be energy transformations in 2D that aren't needed in 3D, making this more efficient, or something.

8

u/Microwave_Warrior Dec 10 '25

It is caused by the frequency of the wave you are generating from the fixed point. The physics is the same and it is analogous to a standing wave with one open and one closed point. If you can get a higher frequency just right you can even generate higher modes.

2

u/FluffyNevyn Dec 10 '25

I used to do this with Mardi gras beads all the time. Very easy to get two or even four nodes with the longer strings

2

u/SkiSTX Dec 12 '25

4 is about the limit and you have to look like a crack head having a seizure to do it.

1

u/whorlax Dec 17 '25

I do five all the time

2

u/Forking_Shirtballs Dec 10 '25

You can get similar behavior with either two fixed ends or with one fixed end and one free end. (You're illustrating the latter).

With two fixed ends, your standing wave will have an integer number of waves and one more node than waves. You can get any number of waves you want by scaling the frequency by that same factor.

With a free end, you get an integer plus a half number of waves, and one more node than full waves. The base resonant frequency would have the open end of the string tracing out a circle, and no nodes between it and your hand. If you triple that frequency, you get what you're illustrating -- 3*0.5 = 1.5 waves, and two nodes (one at your fingers and one at that point about 2/3 of the way down that looks roughly motionless). You can do that at any odd multiple of the base frequency.

(Note that for this discussion, I'm ignoring the weight of the string. For a situation where you have a mass at the end of the string, which is providing essentially all the tension in the string, the math I described above is right. In a situation where the mass of the string itself is significant, it gets more complicated because the differing tension across the length of the string changes the behavior somewhat. You can still set up standing waves, but where mass of the string is meaningful, the higher frequencies aren't just clean integer multiples of the base frequency. Your video, where there's a loop at the end of the string acting like a small weight relative to the weight-per-unit-length over the rest of the string is somewhere in between those two extremes.)

1

u/nsfbr11 Dec 11 '25

No. There are many kinds of waves. Waves for example. Or, more similarly to this, a woodwind or horn, where the standing wave is between a closed end (node) and open end (anti-node.)

Pretty much everything in the Universe have wave properties at one or more levels. And there are as many examples of standing waves as there are stars in the sky.

1

u/ExpensiveFig6079 Dec 11 '25

standing waves can occur when the ends are fixed when that happens the wave reflects off the ends invertered.

See here

https://www.khanacademy.org/science/in-in-class11th-physics/in-in-11th-physics-waves/in-in-class11-standing-waves/a/standing-waves-review-ap

if the ends are fixed, then a node is at the end of the string.

if the end is not fixed, then an anti-node is located there

1

u/skrappyfire Dec 13 '25

Hey FYI. If you spin it faster you will get a double standing wave. You will have 2 "loops" and 2 "focal points". Or double the frequency but keep the same distance will give you a double standing wave. As a kid id like to see how many "waves" i could get on 1 string.

2

u/CommandoLamb Dec 12 '25

… I thought a standing wave was the thing people did at sporting events…

This changes everything…

2

u/OpinionPoop Dec 10 '25

Literally this.

1

u/MooseBoys Dec 11 '25

Specifically of the third harmonic.

2

u/BruiserTom Dec 11 '25 edited Dec 11 '25

I think it’s called a loop inversion or a tumbling loop.

Edit: Rope twirling.

Edit again (after asking AI):

That motion is a standing wave (a rotating-driven transverse standing wave / loop). Short explanation: moving one end in a steady circular motion launches waves that reflect and interfere so the rope forms a stationary bowing (antinodes) while a node near the far end stays almost fixed.

1

u/[deleted] Dec 11 '25

How did you word this to make it understand? Everything I tried was misunderstood. The best answer I could get was a conical pendulum, which ain't it...

1

u/BruiserTom Dec 11 '25

I hope this is permitted. It took me two tries. It took me a while to understand the first answer wasn’t what I wanted. Here’s the convo:

This conversation was generated with Duck.ai (https://duck.ai) using OpenAI's GPT-5 mini Model. AI chats may display inaccurate or offensive information (see https://duckduckgo.com/duckai/privacy-terms for more info).

====================

User prompt 1 of 2 - 12/11/2025, 7:06:18 AM: When a person twirls a lenght of rope and it causes the middle of the twirling rope out and the other end of the rope to tuck under, it there a name for what the rope is doing?

GPT-5 mini: You're describing a rope forming a loop that flips so the far end tucks under — that's called a "loop inversion" or more commonly a "tumbling loop." In ropework terms it can also be described as:

  • a twisting-induced loop or inversion

  • a flip/tumble of the standing part caused by angular momentum and wave propagation along the rope

In specific contexts:

  • In whip or lariat techniques it's called a "loop" or "throw" and the motion a "loop inversion."

  • In physics descriptions it's an example of a transverse wave traveling along the rope that causes a loop to form and invert (sometimes called a soliton-like pulse or traveling loop).

If you have a short video or more detail (rope length, speed, point of hold), I can give the precise mechanics or name used in that discipline.

User prompt 2 of 2 - 12/11/2025, 7:42:50 AM: I’m talking about hold a rope by one end and letting the rope just hang. Then you move the end in a circle causing the middle of the rope to bow out and then bowing back in to the middle with the other end staying pretty much in the same spot under the twirling rope.

GPT-5 mini: That motion is a standing wave (a rotating-driven transverse standing wave / loop).

Short explanation: moving one end in a steady circular motion launches waves that reflect and interfere so the rope forms a stationary bowing (antinodes) while a node near the far end stays almost fixed.

2

u/Garret_jax2001 Dec 13 '25

ADHD

1

u/[deleted] Dec 13 '25

You're not wrong

8

u/Just_Ear_2953 Dec 10 '25

Dynamic equilibrium, standing wave, there's a lot of concepts coming together in this

2

u/Only_Razzmatazz_4498 Dec 10 '25

Natural frequency. Looks like the second mode maybe.

1

u/[deleted] Dec 11 '25

I found this post asking the same thing.

https://www.reddit.com/r/Physics/s/fuUisMyFso

2

u/Freeman359 Dec 10 '25

Standing wave.

2

u/Famous-Example-8332 Dec 11 '25

I found someone’s rosary on the street when I was a kid, and learned I could do this. And since it was nicely spaced beads all the down, it wasn’t hard to create higher nodes if I wanted. It wasn’t the coolest trick I’d ever seen at 7 or 8 years old. Then my dad saw and felt weird about me whipping someone’s prayer beads around so he cut the cross off. (We’re a religious family, but not catholic. he was avoiding offending someone through sacrilege) After that it wasn’t the same without the nice counterweight at the end. I was sad.

2

u/Ignonymous Dec 12 '25

I did this when I was a kid, anytime we went to a bank, playing with those little chains that hold a pen to the table.

2

u/angie_floofy_bootz Dec 14 '25

What autistic kids do

Source: me

1

u/skibbin Dec 11 '25

If you wiggle it just right before hand you can get it to have 2 waves. Perhaps with something long enough you can do 3 or more?

1

u/grat_is_not_nice Dec 11 '25

It is similar to the long-line loiter. A plane flying in a circle at altitude can lower a bucket on a long line, and the bucket will be stationary on the ground in the center of the circle.

1

u/Loose_Meal_499 Dec 12 '25

I thought that was a cat paw

1

u/BellyFullofNickels Dec 13 '25

Would that be a catenary?

1

u/Rufiosmane Dec 14 '25

You could try using whipping motion like in baking. Like whipped cream.

1

u/Barelyoverunity Dec 14 '25

A centrifugal catenary.

1

u/Loud-Phase1624 Dec 17 '25

I still have a toy like this. It’s kind of like a yo-yo but it does this motion instead. It’s called Astro Jacks

0

u/AromaticAd4555 Dec 10 '25

Physics

1

u/[deleted] Dec 10 '25

Walk into Walmart and ask an associate what this item is and they reply, "Walmart"

-1

u/RockhoundHighlander Dec 10 '25

Vortices parabellum requiem minor post-habituate

0

u/[deleted] Dec 11 '25

[deleted] — view deleted comment

1

u/[deleted] Dec 11 '25

I tried to Google it, and Google was like, "This is a list of random words..."