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Unformatted text preview: roofner (bar784) – Practice Problems 2 Solutions – Weathers – (17104) 1 This printout should have 36 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 (part 1 of 3) 10.0 points You dip your finger into a pan of water twice each second, producing waves with crests that are separated by 0 . 12 m. Determine the frequency of these water waves. Correct answer: 2 Hz. Explanation: Let : n = 2 vibrations and Δ t = 1 s . f = number of vibrations Δ t = 2 1 s = 2 Hz . 002 (part 2 of 3) 10.0 points Determine the period of these water waves. Correct answer: 0 . 5 s. Explanation: T = 1 f = 1 2 Hz = . 5 s . 003 (part 3 of 3) 10.0 points Determine the speed of these water waves. Correct answer: 0 . 24 m / s. Explanation: Let : λ = 0 . 12 m . v = f λ = (2 Hz)(0 . 12 m) = . 24 m / s . keywords: 004 10.0 points The distance between two successive crests of a certain transverse wave is 1 . 26 m. Eight crests pass a given point along the direction of travel every 10 . 5 s. Calculate the wave speed. Correct answer: 0 . 96 m / s. Explanation: Let : λ = 1 . 26 m , n = 8 crests , and Δ t = 10 . 5 s . v = f λ = n Δ t λ = 8 10 . 5 s (1 . 26 m) = . 96 m / s . keywords: 005 10.0 points Earthquakes produce two kinds of seismic waves: The 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 v P = 9560 m / s while the S waves travel at a slower speed of v S = 4960 m / s. Suppose a seismic station detects a P wave and then Δ t = 93 . 3 s later detects an S wave. How far away is the earthquake center? Correct answer: 961 . 753 km. Explanation: 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, respectively 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 . roofner (bar784) – Practice Problems 2 Solutions – Weathers – (17104) 2 Consequently, given this time difference and the two waves’ speeds v P and v S , we find the earthquake center being d = v P v S Δ t v P − v S = 961 . 753 km . away from the seismic station. keywords: 006 10.0 points A wave is sent along a long spring by mov ing the left end rapidly to the right and keep ing it there. The figure shows the wave pulse at QR  part RS of the long spring is as yet undisturbed....
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This document was uploaded on 10/26/2011 for the course PHYS 1710 at North Texas.
 Fall '08
 Weathers
 mechanics

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