I recently bought a small Slinky spring so as to observe the speed of a wave along its length. I stretched it out quite tightly in a HORIZONTAL position on a spring holder and then plucked at it on one end.

The speed of the wave was incredibly fast as it bounced from one end to the other, eventually dying out. Then I tried the same thing again but with the spring in a VERTICAL position, believing that the force of gravity would have some type of effect on the wave speed.

However, due to the extremely fast speed of the waves, I could not determine whether there was any effect by the force of gravity. Yes, I tried the experiment with the spring stretched out loosely, but then the wave action was very mushy and lethargic, leading nowhere.

I tentatively believe that gravity will affect a rebounding, vertically-oriented traveling wave somehow, probably by assisting its speed downward and then detracting from its speed upwards, making the horizontal or vertical position of the spring irrelevant on the speed of the wave. But right now, this is all conjecture on my part. Therefore: Help please.

  • $\begingroup$ Question: When you stretched it out vertically, did you fix both ends of it or did you just let the bottom end hang freely? $\endgroup$ – tmwilson26 Oct 21 '15 at 17:08
  • $\begingroup$ The wave doesn't weigh anything. Gravity does not directly affect the wave, but it might have an effect none the less. The tension in the spring will be approximately constant when the spring is stretched horizontally, but in the vertical configuration, the tension will vary along the length as a function of height. If the tension affects the speed of the wave... (But I forget, does it?) $\endgroup$ – Solomon Slow Oct 21 '15 at 17:25
  • $\begingroup$ tmwilson26: Fixed tightly at both ends on a dedicated spring holder. Entire apparatus can be aligned either horizontally or vertically. In this experiment, only changed the alignment from horizontal to vertical. Thanks! $\endgroup$ – Eddie Oct 23 '15 at 17:25
  • $\begingroup$ james large: Yes, I am sure you are correct, as I too believe the lower half of the vertically oriented spring will compress slightly more than the upper, as a function of spring's own weight pressing down on it. But it can't be very much of a difference, maybe even negligible. Thanks! $\endgroup$ – Eddie Oct 23 '15 at 17:28
  • $\begingroup$ @Jordan Sharp: Thanks for a most complete explanation. Now, is there an equation for a "wave on a spring" that I can use for further study of spring wave speed, momentum, force, etc? Thanks! $\endgroup$ – Eddie Oct 23 '15 at 17:34

For the force of gravity to affect the speed of a wave as it travels through any medium logically requires that the force of gravity either (a) has an effect on the wave itself or (b) affects the properties of that medium in such a way as to affect the propagation of a wave through it.

In this instance, the wave that you observe is a property of the spring. The wave itself does not exist; the spring is said to have the property of oscillating according to a particular wave function. As such, the first hypothesis cannot be true.

The second hypothesis is half true, in as much as it affects the properties of the medium, but not the propagation of the wave itself. Essentially, the magnitude and duration of the wave will both be reduced as gravity is increased (as long as the input energy and all other conditions remain the same) - the spring will be held in place by the gravitational field of the earth and this will quickly cause the oscillations to decay in magnitude as energy is lost, with energy being lost more quickly in a stronger gravitational field, and this always leads to the eventual end of the wave; when the energy runs out. The more intense the gravity, the quicker a spring will lose any input energy and hence the sooner the wave will end.

However, the orientation of the spring in a gravitational field is not going to make a difference in the actual speed at which the wave propagates through the same medium, simply because the wave is caused primarily by the Electromagnetic, Strong nuclear and weak nuclear forces. As such, if it is held in an identical fashion and all else remains the same, there will be no measurable difference between the speed of a wave through a horizontal spring and that of a vertical spring.

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