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sashaice [31]
3 years ago
6

Two traveling sinusoidal waves are described by the wave functions y1 = 4.85 sin [(4.35x − 1270t)] y2 = 4.85 sin [(4.35x − 1270t

− 0.250)] where x, y1, and y2 are in meters and t is in seconds. (a) What is the amplitude of the resultant wave function y1 + y2?
Physics
1 answer:
Tamiku [17]3 years ago
8 0

Answer:

Approximately 9.62.

Explanation:

y_1 = 4.85\, \sin[(4.35\, x - 1270\, t) + 0].

y_2 = 4.85\, \sin[(4.35\, x - 1270\, t) + (-0.250)].

Notice that sine waves y_1 and y_2 share the same frequency and wavelength. The only distinction between these two waves is the (-0.250) in y_2\!.

Therefore, the sum (y_1 + y_2) would still be a sine wave. The amplitude of (y_1 + y_2)\! could be found without using calculus.

Consider the sum-of-angle identity for sine:

\sin(a + b) = \sin(a) \cdot \cos(b) + \cos(a) \cdot \sin(b).

Compare the expression \sin(a + b) to y_2. Let a = (4.35\, x - 1270) and b = (-0.250). Apply the sum-of-angle identity of sine to rewrite y_2\!.

\begin{aligned}y_2 &= 4.85\, \sin[(\underbrace{4.35\, x - 1270\, t}_{a}) + (\underbrace{-0.250}_{b})]\\ &= 4.85 \, [\sin(4.35\, x - 1270\, t)\cdot \cos(-0.250) \\ &\quad\quad\quad\; + \cos(4.35\, x - 1270\, t)\cdot \sin(-0.250)] \end{aligned}.

Therefore, the sum (y_1 + y_2) would become:

\begin{aligned}& y_1 + y_2\\[0.5em] &= 4.85\, [\sin(4.35\, x - 1270\, t) \\ &\quad \quad \quad\;+\sin(4.35\, x - 1270\, t)\cdot \cos(-0.250) \\ &\quad\quad\quad\; + \cos(4.35\, x - 1270\, t)\cdot \sin(-0.250)] \\[0.5em] &= 4.85\, [\sin(4.35\, x - 1270\, t)\cdot (1 + \cos(-0.250)) \\ &\quad\quad\quad\; + \cos(4.35\, x - 1270\, t)\cdot \sin(-0.250)] \end{aligned}.

Consider: would it be possible to find m and c that satisfy the following hypothetical equation?

\begin{aligned}& (4.85\, m)\cdot \sin((4.35\, x - 1270\, t) + c)\\&= 4.85\, [\sin(4.35\, x - 1270\, t)\cdot (1 + \cos(-0.250)) \\ &\quad\quad\quad\; + \cos(4.35\, x - 1270\, t)\cdot \sin(-0.250)] \end{aligned}.

Simplify this hypothetical equation:

\begin{aligned}& m\cdot \sin((4.35\, x - 1270\, t) + c)\\&=\sin(4.35\, x - 1270\, t)\cdot (1 + \cos(-0.250)) \\ &\quad\quad + \cos(4.35\, x - 1270\, t)\cdot \sin(-0.250)\end{aligned}.

Apply the sum-of-angle identity of sine to rewrite the left-hand side:

\begin{aligned}& m\cdot \sin((4.35\, x - 1270\, t) + c)\\[0.5em]&=m\, \sin(4.35\, x - 1270\, t)\cdot \cos(c) \\ &\quad\quad + m\, \cos(4.35\, x - 1270\, t)\cdot \sin(c) \\[0.5em] &=\sin(4.35\, x - 1270\, t)\cdot (m\, \cos(c)) \\ &\quad\quad + \cos(4.35\, x - 1270\, t)\cdot (m\, \sin(c)) \end{aligned}.

Compare this expression with the right-hand side. For this hypothetical equation to hold for all real x and t, the following should be satisfied:

\displaystyle 1 + \cos(-0.250) = m\, \cos(c), and

\displaystyle \sin(-0.250) = m\, \sin(c).

Consider the Pythagorean identity. For any real number a:

{\left(\sin(a)\right)}^{2} + {\left(\cos(a)\right)}^{2} = 1^2.

Make use of the Pythagorean identity to solve this system of equations for m. Square both sides of both equations:

\displaystyle 1 + 2\, \cos(-0.250) +  {\left(\cos(-0.250)\right)}^2= m^2\, {\left(\cos(c)\right)}^2.

\displaystyle {\left(\sin(-0.250)\right)}^{2} = m^2\, {\left(\sin(c)\right)}^2.

Take the sum of these two equations.

Left-hand side:

\begin{aligned}& 1 + 2\, \cos(-0.250) + \underbrace{{\left(\cos(-0.250)\right)}^2 + {\left(\sin(-0.250)\right)}^2}_{1}\\ &= 1 + 2\, \cos(-0.250) + 1 \\ &= 2 + 2\, \cos(-0.250) \end{aligned}.

Right-hand side:

\begin{aligned} &m^2\, {\left(\cos(c)\right)}^2 + m^2\, {\left(\sin(c)\right)}^2 \\ &= m^2\, \left( {\left(\sin(c)\right)}^2 +  {\left(\cos(c)\right)}^2\right)\\ &= m^2\end{aligned}.

Therefore:

m^2 = 2 + 2\, \cos(-0.250).

m = \sqrt{2 + 2\, \cos(-0.250)} \approx 1.98.

Substitute m = \sqrt{2 + 2\, \cos(-0.250)} back to the system to find c. However, notice that the exact value of c\! isn't required for finding the amplitude of (y_1 + y_2) = (4.85\, m)\cdot \sin((4.35\, x - 1270\, t) + c).

(Side note: one possible value of c is \displaystyle \arccos\left(\frac{1 + \cos(0.250)}{\sqrt{2 \times (1 + \cos(0.250))}}\right) \approx 0.125 radians.)

As long as \! c is a real number, the amplitude of (y_1 + y_2) = (4.85\, m)\cdot \sin((4.35\, x - 1270\, t) + c) would be equal to the absolute value of (4.85\, m).

Therefore, the amplitude of (y_1 + y_2) would be:

\begin{aligned}|4.85\, m| &= 4.85 \times \sqrt{2 + 2\, \cos(-0.250)} \\&\approx 9.62 \end{aligned}.

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