# Wave interference and resolving resultants

1. May 8, 2006

### kel

Me again ! :surprised

I have the following question and this time I really have no idea!

2 sinusoidal waves, identical except for their phase, travel in the same direction along a stretched string and interfere to produce a resultant wave given by:

x(z,t) = 3Cos (20z-4t+0.82)

Where x is in mm, z in metres and t in seconds

a- What is the wavelength of the 2 original waves
b- What is the phase difference between them
c- What is their amplitude

At the present I'm stuck on 'a' and may/may not be able to do b and c once I know how to work out 'a'.

I can see how to add original waves, but how do I go about resolving the resultant wave? I guess it's similar to vector algebra, but mmmmm.... really don't know where to begin on this one.

Any help would be very much appreciated.

2. May 8, 2006

### Päällikkö

This should get you started:
$$\sin \alpha + \sin \beta = 2 \cos \left( \frac{\alpha - \beta}{2} \right) \sin \left( \frac{\alpha + \beta}{2} \right)$$

3. May 8, 2006

### kel

You'll have to excuse me for being a bit clueless here, but how do I relate that to the resultant wave?

Sorry, it's late and my brain needs caffine!:zzz:
Cheers

4. May 8, 2006

### Päällikkö

Adding the two waves gives you the resultant wave.

Use the fact that the waves are identical, except for phase, to write equations for the two waves.

5. May 9, 2006

### kel

So far, I've got the following

y(z,t) = 3cos(20z-4t) + 3cos(20z-4t+0.82)

but I don't think this is right

6. May 9, 2006

### Päällikkö

The two waves, identical except for phase:
$$y(z,t) = A\cos(kz - \omega t + \phi_1) + A\cos(kz - \omega t + \phi_2)$$

The earlier equation for cosines:
$$\cos \alpha + \cos \beta = 2 \cos \left( \frac{\alpha - \beta}{2} \right) \cos \left( \frac{\alpha + \beta}{2} \right)$$

Can you now see where we're going?

7. May 9, 2006

### kel

So, I'd get

$$\cos \alpha + \cos \beta = 2 \cos \left( \frac{\0 - \0.82}{2} \right) \cos \left( \frac{\0.82}{2} \right)$$
Which leaves
$$\cos \alpha + \cos \beta = 2 \cos \left(-0.41\right) \cos \left(0.41\right)$$

Last edited: May 9, 2006
8. May 9, 2006

### Päällikkö

I get:
$$y(z,t) = A\cos(kz - \omega t + \phi_1) + A\cos(kz - \omega t + \phi_2)$$
$$y(z,t) = 2A \cos\left(\frac{\phi_1-\phi_2}{2}\right) \cos\left(\frac{2kz - 2\omega t + \phi_1 + \phi_2}{2}\right)$$

$$y(z,t) = A' \cos \left( kz - \omega t + \frac{\phi_1 + \phi_2}{2} \right)$$ , where
$$A' = 2A \cos \left(\frac{\phi_1-\phi_2}{2}\right)$$
Now this looks quite a bit like the equation given in the problem.

Last edited: May 9, 2006
9. May 9, 2006

### kel

I tried it like this (adding two waves together):

a*sin(kx-wt+PI/2) + a*sin(kx-wt+PHI+PI/2) ---> a*cos(kx-wt) +
a*cos(kx-wt+PHI).

adding these using the trig identities you get:
2a*cos(kx-wt+PHI/2)*cos(-PHI/2).

comparing this with the resultant wave, then:
k = 20
w = 4 and
PHI/2 = 0.82.

and using k = 2PI/Lambda and w=2P

wave 2a = 3 this would suggest that the amplitude is 3/2.

10. May 9, 2006

### Päällikkö

Good .
The thing I'll have to disagree about is the amplitude. The latter cosine term, cos(-PHI/2) = cos(PHI/2), contributes into the amplitude (see A' in my previous post).

Last edited: May 9, 2006