# Reappearing waves

1. Jan 26, 2013

### statement

Hi!
When two equal waves meet in a destructive interference a short while the resultant wave is flat. What makes the waves reappear?

2. Jan 26, 2013

### phinds

I see what you mean, but they don't "REappear" because they have never disappeared they have just had a momentary interaction that results in zero amplitude.

3. Jan 26, 2013

### BenG549

Do you have an example of this. As far as I am aware if two wave of equal magnitude and opposite phase destructively interfere with each other the waves do not spontaneously "reappear".

4. Jan 26, 2013

### statement

5. Jan 26, 2013

### Simon Bridge

As phinds pointed out - the motion of the particles returns because the destructively interfering waves have moved past each other.

Consider - a ball, bouncing, is stationary in the instant that it strikes the surface - then, in the next instant, the motion reappears (in the opposite direction). How did this happen?

Simplistically: In the pulse on a string - what happens is that there are two forces displacing the particles. They are equal and opposite but they act in different places so you see different pulses. They move towards each other - when they act in the same place, the forces oppose each other directly for no net result, but the thing providing the two forces has not gone away - they are both still pulling on the particles. When the forces move past each other they are no longer acting on the same particle, so the displacement returns.

6. Jan 27, 2013

### statement

Thanks!

7. Jan 27, 2013

### sophiecentaur

In mechanical waves of any sort, there is a constant flow of energy because any mechanism that's supporting a mechanical wave can be looked upon as a series of masses, joined by springs (or the equivalent) and there is a combination of Potential and Kinetic energies. When the displacement happens to be zero (zero PE), the velocity (KE) is at a maximum. So there is still energy flow even though the displacements in the two interfering waves appears to be zero. The energy hasn't 'disappeared' anywhere - you just don't recognise it's there in the nodes, where the two potential energies add together.