Phys123_2021-22_Q3_Final

Name ___________________________________________UW net ID ____________________ last first Phys 123A Final exam 1 I. Lecture Multiple choice (70 points – 21 questions) 1) (3 pts) An object with mass ? is attached to a vertical spring with spring constant 𝑘 . The object undergoes simple harmonic motion with maximum displacement ? and maximum speed ? 1 . Now the spring is replaced, and the new spring constant is 0.25𝑘 and the object undergoes simple harmonic motion with maximum displacement ?/2 and maximum speed ? 2 . What is the ratio ? 1 / ? 2 ? A. ? 1 / ? 2 = 4 B. ? 1 / ? 2 = 2 C. ? 1 / ? 2 = 1 D. ? 1 / ? 2 = 0.5 E. ? 1 / ? 2 = 0.25 2) (3 pts) The pendulum outside the classroom has a period of 7.9 s. Now it is moved to the surface of Jupiter where the gravity is 2.5 times stronger than the gravity on the surface of the earth. What is the new period? A. 19.8 s B. 12.5 s C. 7.9 s D. 5.0 s E. 3.2 s 3) (3 pts) A traveling pulse is described by the time-dependent wave function ?(?, 𝑡) = 1 ? 2 +(?𝑥−?𝑡) 2 where ? = 5.0 m -0.5 ; ? = 0.20 m -1.5 ; ? = 0.5 m -0.5 s -1 . What is the wave speed of this pulse? A. 0.10 m/s B. 0.50 m/s C. 2.5 m/s D. 5.0 m/s E. 12.5 m/s

2 4) (3 pts) Two different guitar strings are under the same tension with the same length between fixed ends. If you pluck the strings in exactly the same way, string 1 produces a lower pitched note than string 2. What can you conclude about the wave speeds in strings 1, 𝑐 1 , and 2, 𝑐 2 , and the wavelengths in strings 1, 𝜆 1 , and 2, 𝜆 2 ? A. 𝑐 1 = 𝑐 2 and 𝜆 1 = 𝜆 2 B. 𝑐 1 < 𝑐 2 and 𝜆 1 = 𝜆 2 C. 𝑐 1 > 𝑐 2 and 𝜆 1 = 𝜆 2 D. 𝑐 1 = 𝑐 2 and 𝜆 1 > 𝜆 2 E. 𝑐 1 = 𝑐 2 and 𝜆 1 < 𝜆 2 5) (3 pts) Two coherent sources S 1 and S 2 emit waves with frequency 𝑓 = 300 Hz in a medium with wave speed 𝑐 = 6.0 m/s. In the configuration shown below, point P is on a nodal line, 15 cm away from S 2 . Now suppose that source S 2 is moved along the dashed diagonal line, for what positions of S 2 will P now be on an antinodal line? i. 1.0 cm closer to P. ii. 2.0 cm further away from P. iii. 3.0 cm further away from P A. i. only B. ii. only C. iii. only D. i. and iii. E. ii. and iii. 6) (4 pts) A woman standing on flat ground observes a jet flying parallel to the ground. The jet has a speed of Mach 1.5 (1.5 times the speed of sound). If the altitude of the jet is 10,000 m, how long after the jet passes directly overhead does she hear the sonic boom? Assume that the speed of sound is 343 m/s. A. 17 s B. 19 s C. 22 s D. 29 s E. 33 s

4 10) (4 pts) Two different light sources are incident on a double slit with unknown spacing between slits. One light source has a wavelength of 4.50×10 − 7 m and the other has an unknown wavelength. The interference patterns cast on a screen a long distance away are shown below. What is the unknown wavelength? A. 4.5×10 − 7 m B. 4.8×10 − 7 m C. 7.5×10 − 7 m D. 8.0×10 − 7 m E. More information is required. 11) (3 pts) Red light shines down on a piece of glass at a slight angle to horizontal that rests with the lefthand side on a flat piece of glass as shown at right. There is a small air gap between the pieces of glass. The image below labeled original shows the reflected light. Which of the images shows what we would expect to see if we used light with a slightly longer wavelength?

Name ___________________________________________UW net ID ____________________ last first Phys 123A Final exam 5 12) (3 pts) You fire a beam of neutrons at a crystal with a flat surface and a distance between layers of 4.0 × 10 −10 m. At what speed should you fire the neutrons such that the first Bragg diffraction angle is 24 ° ? Note that the mass of a neutron is ? 𝑛 = 1.7 × 10 −27 kg. A. 5.3 × 10 2 m/s B. 1.1 × 10 3 m/s C. 1.2 × 10 3 m/s D. 2.4 × 10 2 m/s E. There is no neutron speed that could produce a diffraction maximum 13) (4 pts) A non-viscous liquid has laminar flow from left to right through the pipe shown (sizes not to scale). At point A, the pressure of the fluid is 5.0 × 10 5 Pa and the speed of the fluid is 10 m/s. At point B, the speed of the fluid is 20 m/s. The density of the fluid is 1000 kg/m 3 . What is the pressure of the fluid at point B? A. 2.0 × 10 5 Pa B. 3.5 × 10 5 Pa C. 5.0 × 10 5 Pa D. 6.5 × 10 5 Pa E. 8.0 × 10 5 Pa 14) (3 pts) An open U-tube has mercury in it and then water is added to the left-hand side. Water does not mix with mercury and has a lower density than mercury. How does 𝑃 ? , the pressure at point A, compare with 𝑃 ? , the pressure at point B? A. 𝑃 ? > 𝑃 ? B. 𝑃 ? = 𝑃 ? C. 𝑃 ? < 𝑃 ? D. Not enough information 15) (3 pts) One mole of gas atoms (6×10 23 atoms) is taken from a volume of 1L to 2L through an isothermal process. How much has its entropy increased? A. The entropy is the same because the process is isothermal. B. 0.7 C. 2 D. 4.2×10 23 E. 1.4×10 24 A B

Name ___________________________________________UW net ID ____________________ last first Phys 123A Final exam 7 19) (3 pts) Comet 67P is made of 5.7 × 10 12 kg of ice. Far from the sun it is at a temperature of 2.7 K. How much energy would it take to turn the ice into liquid water at 0 ° C? You may need some of the following assumptions: Ice melts at 0 ° C. The heat capacity of ice is 2.1 kJ/kg/K. The heat capacity of water is 4.2 kJ/kg/K. The heat of fusion of ice is 330 kJ/kg. The density of ice is about 1kg/m 3 . A. 3.0×10 15 kJ B. 5.1×10 15 kJ C. 8.1×10 15 kJ D. 1.0×10 16 kJ E. None of the above 20) (3 pts) When building an ideal refrigerator that is supposed to keep your beverage at 5 ° C when it is 30 ° C outside, what is the coefficient of performance of cooling (how many Watts of heat can you pump out of the refrigerator and into the air per Watt of electrical energy consumed)? A. 0.9 B. 1 C. 11 D. 12 E. 30 21) (4 pts) What is the maximum efficiency of extracting electrical work from the temperature difference between a fission reactor (600 K) and a warm river (45 ° C)? A. 0.98 B. 0.96 C. 0.63 D. 0.53 E. 0.47

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10 24) (4 pts) Coherent monochromatic light is incident on a mask with 3 evenly spaced identical slits. The resulting pattern observed on a screen placed some distance from the mask is shown at top right. Point C represents the center of the screen. Which point on the screen corresponds to a point with the phasor diagram at bottom right? A. Point X B. Point Y C. Point Z D. Both points X and Y 25) (4 pts) In the simulation for lab B2 you used a laser with a wavelength of 650 nm and a mask with two slits with slit widths of 0.04 mm. You varied the center-to-center spacing between the slits d and observed the diffraction pattern on a screen placed 1.5 m from the mask. You measured the distance between the central bright spot and the bright spot to the right of it, D , and made the plot from your measurements. Which of the following best matches the conclusion you would reach from your data? A. The trendline is not a good fit as it does not go through all the uncertainty bars. B. The trendline is not a good fit as it goes through more than 2/3rds of the uncertainty bars. C. The trendline is a good fit as it goes through more than 2/3rds of the uncertainty bars. D. The trendline is not a good fit as it goes through less than 2/3rds of the uncertainty bars. E. The trendline is a good fit as it goes through less than 2/3rds of the uncertainty bars.

Name ___________________________________________UW net ID ____________________ last first Phys 123A Final exam 11 III. Tutorial multiple choice (15 points – 5 questions) 26) (3 pts) Two springs are joined at a junction J. A pulse is generated at one end of the springs, and the image below shows the shape of the springs after the pulse has reached the boundary between the springs. Which of the following statements are true? i. The tension is greater in the left spring. ii. The mass per unit length is greater in the right spring. iii. The pulse was generated in the right spring. A. i. only B. i. and ii. C. i. and iii. D. ii. and iii. E. All of the statements are true. 27) (3 pts) An ideal gas at room temperature is confined to a cylindrical chamber with a moveable piston. A mass is placed on the piston such that it applies a constant force on the piston. With the constant force applied to the piston the cylinder is placed into ice cold water and reaches thermal equilibrium. Which of the following decrease after the cylinder is placed in ice cold water? i. The temperature of the gas. ii. The volume of the gas. iii. The pressure of the gas. A. i. only B. iii. only C. i. and ii. D. i. and iii. E. All of the above increase.

Name ___________________________________________UW net ID ____________________ last first Phys 123A Final exam 13 30) (3 pts) A heat engine operates in a cycle during which the working substance passes through a series of thermodynamic processes returning repeatedly to each state. During a single cycle we can consider the change in entropy of the substance, ∆𝑆 int , and the change in entropy of the environment, ∆𝑆 env . For a single cycle of an engine that does not violate the 2 nd law of thermodynamics, which of the following statements must be true? i. ∆𝑆 int + ∆𝑆 env ≥ 0 ii. ∆𝑆 int = 0 iii. ∆𝑆 env = 0 A. i. only B. ii. only C. iii. only D. i. and ii. E. i. and iii.