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Practice%20Final%20Exam%20solutions

# Practice%20Final%20Exam%20solutions - 1 1070 Practice Final...

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1 1070 Practice Final Exam Solutions W’08 1. Travelling sound wave in +x direction Forms for travelling wave are: y= y 0 s in (2π ) = y 0 s in [ω(t − )], ω = 2π f = Can only be (C) or (D) Angular frequency ω= 2πf, f = f = = 2000 Hz ω = 4000 π = = where v = 340 y = y 0 s in [4000 π (t - ) ] ANS (C) 2. “Intensity level”, IL at 15m 0 IL = 10 og I I Given as 85 db 0 I I = ℓ og -1 (8.5) I = I 0 (10 8.5 ) The s tandard reference intensity I 0 = 10 -12 w/m 2

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2 I = 10 -12 10 8.5 = 10 -3.5 = 10 +0.5 10 -4 = x 10 -4 = 3.16 x 10 -4 w/m 2 “Power” = IA where ( A =area) A = 2.0 1.0 m 2 = 2.0 m 2 P = (3.16 10 -4 ) 2.0 = 6.32 10 -4 w ANS (A) (note, the distance 15.0 m is irrelevant ) 3. “Beat Frequencies” If two waves of frequency f 1 and f 2 are added, the beat frequency f B is f B = | f 1 f 2 | (Abs. value is not in text, but is in lab manual) 5 = | f string - 440| = | f string 450 | The only common solution is f string = 445, hence the 2 nd harmonic is at 2(445) = 890 Hz, ANS (B) 4. Clearly, only (D) is false 5. Lens r 1 r 2 oil ( n 2 ) light water ( n 1 ) By conventions, both r 1 and r 2 are positive.
3 The “Lensmaker Equation” is P = ( ) ( ) = ( ) ( ) = + = 2.9

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