Why Doesn't Gravity Act as a Restoring Force in Wave Pulses?

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Discussion Overview

The discussion revolves around the role of gravity as a potential restoring force in wave motion, particularly in the context of sound waves in air and mechanical waves in mediums like slinkies and ropes. Participants explore the conditions necessary for wave formation and the nature of forces involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that a rhythmic disturbance from a hand can create a wave in a slinky, questioning why gravity does not similarly disturb air molecules to create sound or wave motion.
  • Another participant mentions that gravitational fields can create gravitational waves, but these are weak and difficult to detect compared to other forces.
  • A participant queries whether gravity, despite being weak, can cause wave motion among air molecules in Earth's atmosphere, drawing a comparison to the force applied to a slinky.
  • It is noted that the force of gravity on air molecules is constant, while the force applied to the slinky is variable, suggesting that a restoring force is necessary for wave formation.
  • A participant expresses confusion about the formation of waves and seeks clarification on whether gravity acts as a restoring force in water waves or in guitar strings.
  • Another participant confirms that the restoring force of gravity in water waves or tension in guitar strings contributes to oscillation patterns that can be represented sinusoidally.
  • A participant questions why gravity does not act as a restoring force to return a wave pulse in a rope to equilibrium, noting that the pulse remains unchanged as it travels through the medium.

Areas of Agreement / Disagreement

Participants generally agree on the necessity of a restoring force for wave formation, but there is ongoing debate regarding the role of gravity in this context, particularly in relation to air molecules and wave pulses in ropes.

Contextual Notes

Participants discuss the nature of forces and their effects on wave motion, but there are unresolved questions about the specific conditions under which gravity might act as a restoring force, as well as the implications for different mediums.

Sammy101
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When you move your hand with a slinky in it, the slinky begins to move because you gave energy from your hand to the slinky. A wave is not always formed, but when the disturbance from your hand is rhythmic, a wave will form. Is this right so far?

A force acted on the slinky and caused a disturbance. Gravity is a force that acts on all objects with mass including air molecules. Therefore why does gravity not create a disturbance on air molecules and cause sound or movement as in a wave?
 
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Yeah, gravitational fields do create gravitational waves. They're just not very detectible since gravitational forces are so weak compared to anything else; especially if you're comparing the gravitational force of a binary star system hundreds of light years away to the electrostatic interactions between two molecules.

It is possible to detect gravitational waves though;
http://en.wikipedia.org/wiki/LIGO
 
Hey thanks for the response! I am really interested in what you are saying but I think what I am trying to get at is that if a force as small as that coming from my hand can move a spring then can a force like gravity (which I know is not very strong though) cause wave motion between air molecules in Earths atmosphere for example?
 
Well the problem is that the force due to gravity on the air molecules is constant, while the force that you're applying to the slinky is changing. To have a wave you need a restoring force and some other force. For example, a bouncy ball will create a "wave" of sorts.

But in the case of air molecules just chillin' in the atmosphere, there's nothing to really make them "wave" at all.
 
Thanks! That helps a lot to understand that you need a restoring force to create waves because I was always kinda confused on how waves were actually formed beyond just giving a medium energy or a disturbance.

Is this restoring force of gravity in water waves or tension and gravity in guitar strings what causes waves to oscillate in curves which can be represented by a sinusoidal function?
 
Sammy101 said:
Thanks! That helps a lot to understand that you need a restoring force to create waves because I was always kinda confused on how waves were actually formed beyond just giving a medium energy or a disturbance.

Is this restoring force of gravity in water waves or tension and gravity in guitar strings what causes waves to oscillate in curves which can be represented by a sinusoidal function?

Exactly.
 
okay this is all making sense but when I send a single wave pulse (such as a crest) through a rope for example, why does gravity as a restoring force not push this pulse down to the equilibrium position to the trough and then pull it back up to form a crest?

When we did experiments with a rope or slinky in physics and sent a single pulse, the pulse would remain a crest (or trough) the entire length of the rope. Why does gravity not act as a restoring force here?
 

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