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Unformatted text preview: roofner (bar784) – Practice Problems 2 Solutions – Weathers – (17104) 1 This print-out should have 36 questions. Multiple-choice 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