Does friction slow down a wave (in a phone cord among others)?

[Head_Unit]

I'll pose that as a general question, and specifically for sound waves, which Googling did not answer in a satisfactory way.

And a specific instance I'm concerned with is as follows:
- Students are using telephone cords to make waves (cheaper and much more durable than Slinkies!).
- Most groups hold the cord stretched through the air.
- But one group put the cord on the floor, and snapped it sideways to generate a wave.

Especially at higher amplitudes (more sideways) the cord rubs a lot on the floor. We are debating, to no conclusion: does that friction affects the measured speed of the pulse?

 

[mfb]

I would expect this, but not as a strong effect.

Something that certainly influences the propagation speed is the tension of the cord (if there is tension at all).

 

[AlephZero]

Something that certainly influences the propagation speed is the tension of the cord (if there is tension at all).

There will be tension if the cord was suspended in the air, unless the cord is weightless!

The speed will depend mainly on two things, the tension in the cord and its own flexibility, acting like a coil spring.

There will be smaller effects from the amplitude of the waves (a bigger amplitude causes more "stretching" of the length of the cord) and the amount of damping.

For a damping force proportional to velocity, the frequency is reduced from $\omega$ to $\omega\sqrt{1 - \beta^2}$ where $\beta$ is a measure of the amount of damping. Even if $\beta$ is quite large, for example 0.14 (which would mean the wave amplitude would be halved in about one cycle or one wavelength) the frequency change is only about 1%.

Actually, if the only source of damping is Coulomb friction (a constant force in the opposite direction to the velocity), the friction does not change the frequency at all. But that is probably an over-simplified model of your "cord on the floor" experiment.

 

[mfb]

There will be tension if the cord was suspended in the air, unless the cord is weightless!

Sure, but in the scenario where the cord lies on the floor it can be without tension.

 

[PJC01]

I can provide a response to this question. First, let's define what a wave is. A wave is a disturbance that propagates through a medium, transferring energy from one point to another without the physical movement of the medium itself. In the case of a sound wave, the medium is usually air.

Now, let's consider the role of friction in this scenario. Friction is a force that resists the motion of one surface over another. In the case of the phone cord on the floor, there is definitely friction present as the cord rubs against the floor.

So, does friction slow down a wave? The answer is yes, but it depends on the type of wave and the medium it is traveling through. In the case of a sound wave, friction can affect the measured speed of the pulse. This is because sound waves travel through air, and air molecules experience friction with each other and with any surfaces they come into contact with.

In the specific instance you mentioned, where students are using phone cords to make waves, the presence of friction from the cord rubbing against the floor could affect the measured speed of the wave. This is because the friction between the cord and the floor would dissipate some of the energy of the wave, causing it to slow down.

However, it is important to note that the amount of friction present and its effect on the wave's speed would depend on the properties of the cord and the floor, such as their material and surface texture. In addition, the amplitude of the wave would also play a role in how much friction is present and how much it affects the wave's speed.

In conclusion, friction can indeed slow down a wave, but its effect on the wave's speed would depend on various factors and would need to be carefully considered and measured in any scientific experiment.