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Unformatted text preview: Physics 1112 Spring 2010 University of Georgia Instructor: HBSch¨uttler Solution to PHYS 1112 In-Class Exam #1B Thu. Feb. 4, 2010, 11:00am-12:15pm Conceptual Problems Problem 1: A student runs eastward at 4m/s, away from a vertical plane mirror, while the mirror, mounted on wheels, travels eastward at 7 m/s (with both speeds given relative to the ground). The speed at which the student’s image moves and its direction, relative to the ground, is (A) 10m/s westward (B) 10m/s eastward (C) 11m/s eastward (D) 18m/s westward (E) 18m/s eastward Answer: (B) This is the same problem as HW P02.04, except that the direction of motion of the runner, of the mirror and hence the image have been changed: relative to the ground, the runner is moving in the same direction as the mirror, instead of opposite to (towards) the mirror. The speed of runner relative to ground is +4m / s, and the speed of mirror relative to ground is +7m / s, taking eastward as the positive velocity direction. The speed of runner relative to mirror is thus (4- 7)m / s =- 3m / s which is negative, i.e. , in westward direction. Hence, as seen from the mirror, the runner is actually going westward, approaching the mirror at an absolute speed of 3m / s, while, relative to the ground the mirror is catching up to the runner at that 3m / s relative speed. By law of reflection, the image’s velocity relative to mirror is the same in magnitude as the runner’s, but opposite to the runners direction, i.e. , +3m / s relative to mirror. So, the image, as seen by an observer traveling with the mirror, is going eastward at an absolute speed of 3m / s. But the mirror is going eastward at 7m / s relative to the ground. As seen by a stationary oberver on the ground, the velocity of the image is thus (+3 + 7)m / s = +10m / s. So, from the ground, the image is seen going 10m / s in eastward direction. Problem 2: Sound waves (including ultrasound) have a speed of wave propagation v Air = 346m/s in air and v Water = 1497m/s in water. Also, assume that sin(13 . 364 o ) = 346 / 1497 . A narrow ultrasound beam striking the flat water surface of your swimming pool (A) will undergo total internal reflection if incident from below the water surface with an angle of incidence of 19 . o ; (B) will undergo total internal reflection if incident from below the water surface with an angle of incidence of 9 . 5 o ; 1 Physics 1112 Spring 2010 University of Georgia Instructor: HBSch¨uttler (C) will undergo total internal reflection if incident from above the water surface with an angle of incidence of 9 . 5 o ; (D) will always have an angle of refraction not exceeding 13 . 364 o if the beam is incident from above the water surface; (E) will have an angle of refraction greater than the angle of incidence if the beam is incident from above the water surface without total internal reflection....
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This note was uploaded on 04/23/2010 for the course PHYS 1112L taught by Professor Staff during the Spring '10 term at UGA.
- Spring '10