I work as an elevator constructor. part of an installation involves installing a steel tape the entire length of the shaft. it is very thin (imagine steel duct tape) and is under spring tension to keep it from moving too much. when at the top of the shaft i pulled the tape and sent a wave down the tape. it distinctly bounces up and down the entire length of the shaft a few times and makes a really wierd star wars like sound. it starts out high pitch but ends as a low bass rumble. is the sound based on the length of the tape before and after the wave? almost like frets on a guitar? also i was wondering if the wave moves faster on the way down and slower up due to gravity?
2006-06-25 05:17:26 · 5 answers · asked by dennis r 1 in Science & Mathematics ➔ Physics
I'm not 100% sure but I think your idea is correct. Longitudinal sound waves in the metal, which travel at a different and faster speed than the visible wave, reflect off the pulse or kink (because there's a discontinuity in sound speed at that point). Because the pulse is moving away, the reflected wave has a lowered frequency each time it's reflected - that's expected behavior.
An equivalent way of saying the same thing (although it seems different) is that the fundamental wavelength of the standing wave based on the length of the tape between the two endpoints: the top of the tape and the pulse, is continuously lengthening or shrinking causing the pitch to go down or up continuously like the pitch of a guitar string when the hand slides along the fingerboard.
This can't be a case of a Doppler shift caused by a moving sound source - because in that situation, the pitch would be lowered (for a source travelling directly away) but not continuously decreasing.
With respect to the other question:
It's quite certain that there can't be any difference in speed between a wave moving up or down (at a given point on the tape). The only possible effect of gravity is as explained by Epidavros, to gradually increase the wave speed at higher levels - *if* it so happens that total weight of the tape (e.g. when it's rolled up) is comparable to the spring tension.
2006-06-25 08:31:02 · answer #1 · answered by shimrod 4 · 0⤊ 0⤋
The reason for the strange behaviour is that the tape is under tension due to gravity. The higher from the bottom you go up the tape then the more weight there is below you and so the higher the tension it creates. Hence the tape is under varying tension, and as the speed of a wave on the string varies as the square root of the tension so will the speed.
This varying speed will have two effects. Firstly, the wavelength of a wave travelling on the tape will get shorter as it moves to the top. Second, wave tend to reflect from boundaries where the speed changes, so the wave will tend to propagate oddly in both directions.
2006-06-25 12:31:32 · answer #2 · answered by Epidavros 4 · 0⤊ 0⤋
I would think the changing pitch is due to Doppler shift. It's the same effect that makes the pitch of a car's engine on the highway drop as the car passes you. Since the vibration is travelling down a tensed steel band, it would travel very fast and have a pronounced Doppler effect.
I don't think the speed of the wave would be affected by its direction of travel, up or down. The wave is actually a transverse wave, so the motion of mass is essentially the same whether the wave is travelling up or down. The wave itself has no mass so it is not affected by gravity.
Hope this helps!
2006-06-25 12:34:00 · answer #3 · answered by poorcocoboiboi 6 · 0⤊ 0⤋
I don't know physics. But I do know that the star wars sound effect WAS created by banging on a metal cable!
2006-06-25 12:21:31 · answer #4 · answered by Anonymous · 0⤊ 0⤋
no
2006-06-25 13:26:18 · answer #5 · answered by Thunder 1 · 0⤊ 0⤋