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

  • Context: High School 
  • Thread starter Thread starter Head_Unit
  • Start date Start date
  • Tags Tags
    Friction Wave
Click For Summary

Discussion Overview

The discussion revolves around the impact of friction on the speed of wave propagation in a telephone cord, particularly in the context of sound waves. Participants explore the effects of tension, amplitude, and damping forces on wave speed, with a specific focus on an experimental setup where a cord is placed on the floor versus being suspended in the air.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant poses a general question about the effect of friction on wave speed, specifically in the context of sound waves generated by a telephone cord.
  • Another participant suggests that while friction may have some effect, it is not expected to be strong, emphasizing that tension in the cord is a significant factor influencing wave speed.
  • A participant elaborates that wave speed depends mainly on the tension and flexibility of the cord, with additional minor effects from wave amplitude and damping.
  • It is noted that a damping force proportional to velocity reduces frequency slightly, while Coulomb friction may not affect frequency at all, although this might oversimplify the experimental scenario.
  • One participant highlights that if the cord lies on the floor, it may not have any tension, which could influence the wave propagation differently than when suspended.

Areas of Agreement / Disagreement

Participants express differing views on the extent to which friction affects wave speed, with some agreeing that tension is a primary factor while others question the assumptions about tension in the specific experimental setup. The discussion remains unresolved regarding the overall impact of friction.

Contextual Notes

Participants acknowledge various factors influencing wave speed, such as tension, flexibility, amplitude, and damping, but do not reach a consensus on how these interact with friction in the specific context of the cord on the floor.

Head_Unit
Messages
43
Reaction score
2
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?
 
Physics news on Phys.org
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).
 
mfb said:
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.
 
AlephZero said:
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.