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Unformatted text preview: Physics 1112 Spring 2010 University of Georgia Instructor: HBSchuttler Solutions to Conceptual Practice Problems PHYS 1112 In-Class Exam #1A+1B Thu. Feb. 4, 2010, 9:30am-10:45am and 11:00am-12:15pm CP 1.01: A student runs westward at 3m/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 students image moves and its direction, relative to the ground, is (A) 4m/s westward (B) 4m/s eastward (C) 10m/s eastward (D) 17m/s eastward (E) 17m/s westward Answer: (D) 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 has been reversed: both runner and image are mov- ing away from the mirror instead of towards it. The speed of runner relative to mirror is (3 + 7)m / s = 10m / s in westward direction. Hence, the images speed is also 10m / s, but in eastward direction, relative to the mirror. Since the mirror also moves eastward at 7m / s relative to the ground, the speed of mirror relative to ground and of image relative to mirror add up, to (10 + 7)m / s = 17m / s eastward. CP 1.02: Sound waves (including ultrasound) have a speed of wave propagation v Air = 346m/s in air and v Water = 1497m/s in water. Also, note that sin(13 . 364 o ) = 346 / 1497 . A narrow ultrasound beam striking the flat water surface of your swimming pool (A) will have an angle of refraction smaller than the angle of incidence if the beam is incident from above the water surface; (B) will have an angle of refraction greater than the angle of incidence if the beam is incident from below the water surface; (C) cannot undergo total internal reflection if incident from above the water surface, regardless of the angle of incidence; (D) will undergo total internal reflection if incident from above the water surface with an angle of incidence of 30 o (E) will undergo total internal reflection if incident from below the water surface with an angle of incidence of 30 o . 1 Physics 1112 Spring 2010 University of Georgia Instructor: HBSchuttler Answer: (D) To fully understand the following discussion of the Answer, it is imperative that you make a drawing of the air-water interface, of the normal to that interface, and of the incident and refracted rays, for each case discussed (incidence from above and incidence from below), with correct arrows attached to the rays (indicating their direction of propagation), and with correct relative angular size relations shown (Should angle of incidence be drawn as greater than , or should it be drawn as less than , angle of refraction ?). Refraction of sound at the air-water interface differs from refraction of light at the same interface in that light travels slower in water than in air , whereas sound travels faster in water than in air ....
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