Force on a String: Understanding Constant Tension

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Discussion Overview

The discussion revolves around the concept of tension in a string, particularly in the context of a wave traveling along it and the implications of varying density. Participants explore the conditions under which tension can be considered constant or variable, depending on the orientation and state of the string.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why tension in a string can be considered the same everywhere, particularly in the context of a wave traveling on a string with varying density.
  • Another participant argues that in a horizontal string in equilibrium, the tension must be the same on both sides of a small section, as there is no resultant force acting on it.
  • A different viewpoint suggests that for a vertically hanging string with mass, tension increases towards the top due to the weight supported by the string.
  • One participant challenges the notion of equilibrium in the context of the wave equation, stating that the differential element of the string is not in equilibrium during wave motion, although the tension remains constant in magnitude.
  • A later reply clarifies that the initial statement about equilibrium referred specifically to a static string, not one carrying a wave, and acknowledges the differing tension in a hanging string with mass.
  • Another participant notes that the discussion initially interpreted the query as concerning a vibrating string, highlighting the distinction between static and dynamic systems.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which tension can be considered constant or variable. There is no consensus on the implications of tension in static versus dynamic scenarios, and the discussion remains unresolved regarding the application of these concepts to different types of strings.

Contextual Notes

Participants reference assumptions about the mass of the string and the conditions of equilibrium, which may influence their arguments. The discussion also touches on the mathematical derivation of wave equations, indicating potential complexities in the analysis.

aaaa202
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I just solved a problem where you considered a wave traveling on a string with varying density. I did it all correct but during the problem, I wondered why you can say that the tension in the string is the same everywhere on it. Why is that?
 
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If the string is horizontal and you consider a small section of string it is in equilibrium. It is not accelerating and therefore there is no resultant force. This means the force to the right and the force to the left (ie the tension) on each sectionof string must be the same.
If the string is hanging vertically and the string has mass distributed along its length then the tension increases towards the top of the string. Imagine the string as a series of weights connected together. The top of the string has to support more weight than the bottom of the string.
 
If the string is horizontal and you consider a small section of string it is in equilibrium. It is not accelerating and therefore there is no resultant force.

Are you sure you mean this?

The mathematical derivation of the wave equation for a vibrating string relies on the differential element of string not being in equilibrium.

However the tension is (by definition for a string) always parallel to the direction of the string. It is the change of direction (rotation) of the element which occurs as the wave passes that gives rise to zero net acceration in the x direction but a real variable acceleration in the y direction. This is what causes the element to move up an down with time. Tension is a vector which has magnitude and direction. The magnitude does not change but the direction does.

go well
 
Yes Studiot...I was referring to a static string lying flat...not the string carrying a wave.
I should have made that clear, the extra point I wanted to add was that a hanging string will have differing tension along it if it has mass. In many (most) examples with weights on strings over pulleys etc it is assumed that the string has no mass.
You are quite correct.
Cheers
 
I understood the query to be about the tension in a string carrying a wave, ie a vibrating string.

But yes, certainly a simple string stretched by its own weight or other load is often a static equilibrium system. Catapault action for instance is not.
 

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