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Unformatted text preview: yang (ey942) – HW12 – TSOI – (58160) 1 This print-out should have 22 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Radio waves travel at the speed of light: 3 × 10 5 km / s. What is the wavelength of radio waves re- ceived at 107 . 1 MHz on your FM radio dial? Correct answer: 2 . 80112 m. Explanation: Let : v = 3 × 10 5 km / s = 300 million m / s and f = 107 . 1 MHz = 107 . 1 million Hz . λ = v f = 300 million m / s 107 . 1 million Hz = 2 . 80112 m . 002 10.0 points Earthquakes produce two kinds of seismic waves: he longitudinal primary waves (called P waves) and the transverse secondary waves (called S waves). Both S waves and P waves travel through Earth’s crust and mantle, but at different speeds; the P waves are always faster than the S waves, but their exact speeds depend on depth and location. For the pur- pose of this exercise, we assume the P wave’s speed to be 9930 m / s while the S waves travel at a slower speed of 5230 m / s. If a seismic station detects a P wave and then 96 . 2 s later detects an S wave, how far away is the earthquake center? Correct answer: 1062 . 99 km. Explanation: Let : v P = 9930 m / s , v S = 5230 m / s , and Δ t = 96 . 2 s . Suppose the earthquake happens at time t = 0 at some distance d . The P wave and the S wave are both emitted at the same time t = 0, but they arrive at different times, re- spectively t P = d v P and t S = d v S . The S wave is slower, so it arrives later than the P wave, the time difference being Δ t = d v S- d v P = d ( v P- v S ) v P v S . Consequently, given this time difference and the two waves’ speeds v P and v S , the earth- quake center is d = v P v S Δ t v P- v S = (9930 m / s) (5230 m / s) (96 . 2 s) 9930 m / s- 5230 m / s · 1 km 1000 m = 1062 . 99 km away from the seismic station. 003 (part 1 of 2) 10.0 points A wave moving along the x axis is described by y ( x,t ) = Ae − ( x + v t ) 2 /b where x is in meters, t is in seconds, A = 4 m, v = 5 m / s, and b = 5 m 2 . Determine the speed of the wave. Correct answer: 5 m / s. Explanation: y ( x,t ) = Ae − ( x + v t ) 2 /b is of the form y ( x,t ) = f ( x + v t ) so it describes a wave at v = 5 m / s . 004 (part 2 of 2) 10.0 points Determine the direction of the wave. 1. + y direction 2. + x direction 3.- y direction yang (ey942) – HW12 – TSOI – (58160) 2 4.- x direction correct Explanation: f ( x + v t ) = f ( x- (- v ) t ) so the wave is moving at- 5 m / s (in the negative direction). 005 (part 1 of 2) 10.0 points The time interval indicated on this diagram is G . + S- S S t (seconds)-→ G Which formula corresponds best to the di- agram?...
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This note was uploaded on 11/21/2010 for the course PHY 58160 taught by Professor Tsio during the Fall '10 term at University of Texas.
- Fall '10