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Chapter 11: Problem 34

Two traveling sine waves, identical except for a phase difference \(\phi,\) add so that their superposition produces another traveling wave with the same amplitude as the two component waves. What is the phase difference between the two waves?

Short Answer

Expert verified
The phase difference between two identical sine waves for maintaining the same amplitude when superposed is given by: $$\phi = 2n\pi$$ where \(n\) is an integer.

Step by step solution

01

Write the equation for the two sine waves

Let the two traveling sine waves be given by: $$y_1 = A\sin(kx - \omega t)$$ $$y_2 = A\sin(kx - \omega t + \phi)$$ where \(A\) is the amplitude, \(k\) is the wave number, \(\omega\) is the angular frequency, and \(\phi\) is the phase difference between the two waves.
02

Write the equation for their superposition

The superposition of the two waves is the sum of the two waves: $$y_{total} = y_1 + y_2 = A\sin(kx - \omega t) + A\sin(kx - \omega t + \phi)$$
03

Use the sine addition formula to simplify the superposition equation

To simplify the equation for the total wave, we can use the sine addition formula: $$\sin(a + b) = \sin a \cos b + \cos a \sin b$$ Applying this formula to our superposition equation, we get: $$y_{total} = A[\sin(kx - \omega t)\cos\phi + \cos(kx - \omega t)\sin\phi]$$
04

Find the condition for the amplitude to remain the same

We want to find the value of \(\phi\) for which the amplitude of the resulting wave \(y_{total}\) remains \(A\). Notice that \(\sin(kx - \omega t)\) and \(\cos(kx - \omega t)\) are functions of \(x\) and \(t\), but \(\phi\) is a constant. So, we want to find a value of \(\phi\) such that the amplitude of the sum remains constant at \(A\) for all values of \(x\) and \(t\). To do this, let's rewrite the total wave equation: $$y_{total} = Y(x,t)\sin(kx - \omega t) + Z(x,t)\cos(kx - \omega t)$$ where \(Y(x,t) = A\cos\phi\) and \(Z(x,t) = A\sin\phi\). We want \(Y(x,t)^2 + Z(x,t)^2\) to be constant for all \(x\) and \(t\). This is only possible if: $$Y(x,t)^2 + Z(x,t)^2 = A^2$$
05

Solve for the phase difference \(\phi\)

Using the expressions for \(Y(x,t)\) and \(Z(x,t)\), we can rewrite the equation as: $$[A\cos\phi]^2 + [A\sin\phi]^2 = A^2$$ Dividing both sides by \(A^2\), we get: $$\cos^2\phi + \sin^2\phi = 1$$ This equation holds true for all values of \(\phi\). However, to find the specific value of \(\phi\) that causes the amplitude to remain the same, we must consider the nature of the sine and cosine functions. Recall that \(\cos\phi\) and \(\sin\phi\) will have their maximum values of 1 when \(\phi\) is an integer multiple of \(2\pi\). So, the phase difference \(\phi\) that will result in the superposition maintaining the same amplitude as the component waves is given by: $$\phi = 2n\pi$$ where \(n\) is an integer.

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Most popular questions from this chapter

In contrast to deep-water waves, shallow ripples on the surface of a pond are due to surface tension. The surface tension \(\gamma\) of water characterizes the restoring force; the mass density \(\rho\) of water characterizes the water's inertia. Use dimensional analysis to determine whether the surface waves are dispersive (the wave speed depends on the wavelength) or non dispersive (their wave speed is independent of wavelength). [Hint: Start by assuming that the wave speed is determined by \(\gamma, \rho,\) and the wavelength \(\lambda .1]\)
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