| | Q: | I understand your explanations so far, but in Physics class we learn that when two identical wave pulses travelling in opposite directions meet, they cancel each other out perfectly.
This would mean the particles in the medium do not move. Does this mean there is no energy transfer? What then, causes the next particles to move since the pulses apparently keep travelling unaffected by the temporary cancelling out.
-Jasmine (age 16)
Seattle, WA, USA | | | A: |
Nice question! I’m really glad that you asked it, because I tried raising exactly the same question in my sophomore physics class a couple of weeks ago, but the students didn’t seem very interested.
You’re talking about waves made of particles- sound waves, for example. Let’s say you’re talking about the type of sound ("longitudinal") that travels in air, with the particles moving back and forth along the same direction that the wave travels.
If the sound pressures from the two waves travelling in opposite directions just cancel, the particle velocities from the two waves add up. If the velocities just cancel, then the pressure changes add up. Either way, although one of those two variables (pressure or velocity) momentarily looks like there’s no wave there, the other variable has all the evidence that the wave is still there.
Mike W.
Another way of looking at it is that the two waves produce what is called a "standing wave pattern". "Nodes" exist where there is little or no action at all. In between the nodes are areas where the air molecules oscillate back and forth at twice the amplitude. The total energy is the same as the sum of the two individual waves. This is more easily visualized by considering transverse waves on a string. The stationary pattern of a vibrating violin string can be mathematically decomposed into two travelling waves going in opposite directions.
Lee H
(published on 03/04/07) |
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