# What causes waves

1. Apr 16, 2015

### Scheuerf

What causes a wave such as a water wave or a wave in a spring? Why do waves have crests and troughs? What causes a pulse? Does it also have crest and troughs?

2. Apr 16, 2015

### jerromyjon

Imbalance. As matter is perturbed from balance towards a crest it tends back towards balance but with momentum it overshoots into a trough and back towards balance, which constitutes a pulse.

3. Apr 16, 2015

### rootone

Energy.

4. Apr 16, 2015

### Scheuerf

Why doesn't a pulse like the one in the image I attached have a trough? Also, why does the pulse reflect with opposite direction with a fixed end and reflect in the same direction with a free end?

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5. Apr 16, 2015

### jerromyjon

That could be like a DC surge or an interference overlap... the questions are rather obscure in nature.

6. Apr 16, 2015

### Staff: Mentor

Sometimes there's no alternative to doing the math.

If we're considering water waves (just for definiteness), then gravity is acting to pull the high spot down... but as the water there is pulled down it has to displace the water underneath it, forcing it out to the sides, which displaces the water next to the peak, forcing it up and down. We can model this situation with a differential equation, the "wave equation" - google for it! - and solve it. Depending on the initial conditions, we'll find that any of the phenomena you mention will result.

(It would not be unusual to find an entire semester of an undergraduate physics program devoted to wave phenomena).

7. Apr 16, 2015

### Scheuerf

Okay I have one final question. Why does a wave dissipate? If you had a pulse traveling through a spring in space would it still dissipate?

8. Apr 16, 2015

### Staff: Mentor

Waves dissipate for several reasons.

First, waves in many mediums are "dispersive", meaning that the speed of the wave varies with the frequency (water waves are dispersive, electromagnetic waves in a vacuum are not). A single nicely peaked wave like the one in the image you attached is actually a superposition of many different waves of different frequencies, so in a dispersive medium the peak will become ever shallower and wider as the different components move at different speeds until the peak becomes too small to notice.

Second, many mediums convert a little bit of energy to heat as the wave back and forth; this is called "damping". All springs have non-zero damping, so a wave travelling through a spring will gradually weaken and eventually disappear leaving only a slightly warmer spring.

Third, the wave only has so much energy, so if propagates in a way that tends to spread it out, it will get weaker. This is what happens with light from a point source like a distant star - the light spreads out in all directions, so the farther away we are from the star, the weaker the part of the wave that reaches us will be.