2019PhysicsBowl物理碗真题及答案.pdf

Divisions 1 however, students answer only 40 questions. Answers should be marked on the answer sheet next to the number corresponding to the question number on the test. Division 1 students will answer only questions 1 40. Numbers 41 100 on the answer sheet should remain blank for all Division 1 students. Division 2 students will answer only questions 11 50. Numbers 1 10 and 51 100 on the answer sheet should remain blank for all Division 2 students. Calculator: A hand-held calculator may be used. Any memory must be cleared of data and programs. Calculators may not be shared. Formulas and constants: Only the formulas and constants provided with the contest may be used. Time limit: 45 minutes. Score: Your score is equal to the number of correct answers (no deduction for incorrect answers). If there are tie scores, the entries will be compared from the end of the test forward until the tie is resolved. Thus, the answers to the last few questions may be important in determining the winner and you should consider them carefully. Good Luck! Copyright 2019, AAPT Division 1 2 Division 1 only 1. Select the smallest value from the choices below (A) 15 x 10-3 (B) 0.15 x 100 (C) 0.00015 x 103 (D) 150 x 10-3 (E) 0.00000015 x 106 2. Related to the historical development of understanding gravity, which is the proper chronological order for the work of these three scientists, from earliest to latest? (A) Cavendish, Galileo, Newton (B) Galileo, Cavendish, Newton (C) Galileo, Newton, Cavendish (D) Newton, Galileo, Cavendish (E) Newton, Cavendish, Galileo 3. An isolated solid metal sphere with a radius R is given a positive charge Q. The electric potential at the surface of the sphere is V. What is the electric potential at a distance of 0.5R from the center of the sphere? (A) zero (B) 0.5V (C) V (D) 2V (E) 4V 4. Consider a situation where the acceleration of an object is always directed perpendicular to its velocity. This means that (A) the object is increasing speed. (B) the object is decreasing speed. (C) the object is not moving. (D) the object is turning. (E) this situation would not be physically possible. 5. The acceleration due to gravity on the Moon is less than the acceleration due to gravity on the Earth. Which of the following is true about the mass and weight of an astronaut on the Moons surface, compared to Earth? (A) Mass is less, weight is the same. (B) Mass is the same, weight is less. (C) Both mass and weight are less. (D) Both mass and weight are the same. (E) Mass is more, weight is less 6. Which one of the following is not equivalent to 2.50 miles? (1.00 mi = 1.61 km = 5280 ft, 1.00 yd = 3.00 ft = 12.0 in.) (A) 1.32 x 104 ft (B) 1.58 x 105 in (C) 4.02 x 103 km (D) 4.40 x 103 yd (E) 4.02 x 105 cm DIVISION 1 STUDENTS START HERE. DIVISION 2 STUDENTS Go to question #11 on page 4. Treat = . for ALL questions. Division 1 only 3 Division 1 only 7. A 100 kg person travels from sea level to an altitude of 5000 m. By how many Newtons does their weight change? (A) 0.8 N (B) 1.2 N (C) 1.6 N (D) 2.0 N (E) 2.4 N 8. A candle, a converging lens and a white screen are placed in a line with the lens between the candle and the screen. A distance of 72 cm separates the candle and screen. As the lens is moved to all points between the candle and the screen, only one focused image of the candle can be made on the screen. What is the focal length of the converging lens? (A) 12 cm (B) 18 cm (C) 24 cm (D) 36 cm (E) It cannot be determined without knowing the location of the lens when the focused image is produced. 9. Standby power (sometimes called vampire power) is the power used by a device that is off but plugged in and in a standby mode. Regulations typically limit this power to 1 Watt. If electricity costs $0.10 per kilowatt hour, then to the nearest order of magnitude, and assuming 1 Watt, how much does it cost to leave a device in standby mode for one year? (A) $0.01 (B) $0.10 (C) $1.00 (D) $10.00 (E) $100.00 10. A student generates a transverse periodic wave on a string. The wave travels away from the student at a constant speed v. Which of the following changes by itself will increase the speed at which the wave travels away from the student? (A) The student could use the same string but increase the frequency at which they generate the wave. (B) The student could use the same string but increase the wavelength of the waves they generate. (C) The student could use the same string but increase the amplitude of the waves they generate. (D) The student could use a string with the same length and tension, but greater linear density (E) The student could use the same string, but placed under greater tension. Divisions 1 gmoongearth so WmoonWearth 6. B or C After converting all to miles, choices B) Let ? ? ? 2 ? ? ? ?; ? ?72 ? ?); ? ? ? ? ? ? ? ? ? ? ? Solving for ? gives 36 cm and solving for we get 18 cm. 9. C ?1.0 ?365 ?24 ? ? ?.? ? ? ? ? 8.76 ?8.76 ? $0.10 ? ? $0.876 10. E For mechanical waves, the speed of the wave depends upon the medium. 11. D The range equation: ? ? ? ? shows that doubling the launch speed (? increases the range by a factor of 4. This equation results from combining the horizontal velocity component of a projectile launched at an angle: ? and using it to solve for the horizontal distance ? ? with the vertical component of a projectile launched at an angle: ? and using it to solve for time in flight: ? ? ? . 12. B When the system reaches equilibrium, the net (centripetal) force will be provided by the spring. ? ?; ? ? ? ?; ? ? ? ? 13. D Based on the units in the constant (k) provided: ? ? ? ? ? . 14. E After being excited and moving to a higher energy level, the electrons return to their original energy level and release the excess energy as light. 15. B The 2018 Nobel Prize in Physics was awarded to Arthur Ashkin, Gerard Mourou, and Donna Strickland “for groundbreaking inventions in the field of laser physics”. 16. D ? ? ? ?; ? ? ? ? ? 2; ? 8.00 ? ? ;120 ? ? ? 2.00 ? ? ; ? ?8.00 ? ? ? 2.00 ? ? ? ? 16.0 ? ? ? 16.0 17. D For a number raised to a power, the uncertainty rule is to multiply the relative uncertainty by the power. 1.96 ? 0.01 ? 1.96 ? 0.51%; ? ? ? ? 31.5 ?; ?3?0.51%? ? 1.53%;31.5 ? 1.53% ? 31.5 ? 0.5 ? 18. B Going up: ? ? ? ? ? ; Going down: ? ? ? ? ? A larger force acting on the block will result in a larger acceleration which will cause a larger change in velocity resulting in a steeper slope on the vt graph. 19. D ? ; Impulse is ; ? ? ?; ? ? ? ? ? ?0.500 ?3.60 ? ?4.80 ? ? 4.20 ? 4.20 20. B For a standing transverse mechanical wave: 3 antinodes = 1.5 and 4 antinodes = 2. v = f and v is constant for waves traveling in the same medium. So, the longer the wavelength, the smaller the frequency. ?.? ? ? ? ? ; ? 16 21. D Magnification (M) is the ratio of the image size (hi) to the object size (ho) or the ratio of the image distance (di) to the object distance (do) according to: ? ? ? ? ? ? ?. Choice (A) would form a virtual, enlarged, positive magnification image; choice (B) would form a virtual, smaller, positive magnification image; choice (C) would form a positive magnification image, and choice (E) will form a positive image of the same size. 22. C Because the two masses are connected by the string, they move as one mass and accelerate due to the net force: ? ? ? ? ? ?6.0 ? 4.0 ? ? ?0.60 ? 0.40 ?; a=2.0 m/s2 23. A ? ? ? ? 2; ? ? ? ? ? 2.45 ? ? ? ? ? ; ? ? ? ? 30.0 ? ?3.0 ?2.45 ? ? ? 22.65 N 24. B Because there is no charge across the capacitor, there is no potential difference across the capacitor. Therefore, the potential at the junction between the 100 resistor and the 300 resistor is the same as the potential at the junction between the 400 resistor and the 200 resistor. As a result, the 100 ohm and 400 ohm resistors can be treated as a parallel combination connected in series to the parallel combination of the 300 and 200 resistors. This gives an equivalent resistance of 200 and a total current of 60 mA. The potential difference across the top two resistors must be equal and their total current must add to 60 mA, resulting in a currents of 48 mA and 12 mA in the 100 and 400 resistors, respectively. Using the same reasoning, the currents in the 300 and 400 resistors are 24 mA and 36 mA, respectively. Using the junction to the left of the capacitor: I100 = Ix + I300, 48 mA = 24 mA + Ix. Ix = 24 mA 25. C At steadystate, no charge flows to or from the capacitor, so any current through the 100 resistor continues to the 300 resistor. The same is true for the 400 resistor and the 200 resistor. The potential drops across the 100 and 300 resistor must total 12 V. Because the current is the same in these resistors, the voltage drops are proportional to the resistances: 3 V for the 100 and 9 V for the 300 . By the same reasoning, the voltage drops across the 400 ohm and 200 ohm are 8 V and 4 V respectively. The resulting potential difference between the junction to the left of the capacitor and the junction to the right of the capacitor is 5 V. Using Q = CV = (15 F)(5 V), gives 75 C. 26. D The PDF version of this problem contained a typo in choice D). It should have been listed as 4.77 x 103 W/m2. Therefore, all students that took the PDF version will receive credit for this problem, regardless of the answer given. This type was corrected for the WebAssign version of the exam and there will be no change to the scoring for those students. ? ?; ? ? ? ? ? ? ? ?.? ? ? 27. D Coulomb (C) measures electric charge, the Farad (F) measures capacitance, the Tesla (T) measures magnetic field, and the Henry (H) measures mutual inductance. 28. D On a banked curve the normal force on the car is no longer vertical and has a horizontal component. This component then acts as the centripetal force on the car while on the bank. ? ?;tan ? ? ? ; ? ; ? ? 29. A ? ? ? ? ? ? ? ; ? ? ? ? ? ? ? ? ; ? 3? 30. C The torque is applied at the gear. ? ? ? ? for a ring. As a result, is 5.93 rad/s2. Applying this acceleration to the wheel for 4.0s gives it an angular speed of 23.7 rad/s according to ? ? . When the angular speed is multiplied by the radius, the resulting linear speed is 7.1 m/s. 31. E Using the sound intensity equation, ? 10 ? ? ;? 1.0 10? ? ? and 60 dB, the Intensity (I) of the speaker is calculated to be 1.0 x 106 W/m2 at 15 m. Using ? ? ? ? 4?, it is found that the power of the speaker is 2.8 x 103 W. At 2.0 m from the speaker, the intensity is 5.57 x 105 W. 32. C The blocks are floating so the buoyant force is equal to the weight of the blocks. ? 0 ? . Because the mass remains the same in either arrangement of the blocks, the water level remains unchanged. 33. B When Car 2 catches and passes Car 1, they will both have the same displacement from the starting point. ? ?. Use ? ? ? ? ? ? for both cars, set the two equations equal to each other, then solve for using the quadratic formula. 34. B Using ? ? ?, Michael Phelps had an average speed of 197.71 ? ? . Because only 0.01 s separated him from the secondplace finisher and using these numbers gives 1.98 cm. 35. A The average kinetic energy of the molecules is directly related to both their mass and the temperature. ? ? ? ? ?=? ? . 36. A Solve for V, I, R4=0.4V, 0.0133A, 0.0053W; R5=0.6V, 0.02A, 0.012W 37. A See resistor values for #36. 38. B The largest acceleration value from the graph is 10.4 m/s2. The difference between this value and the constant velocity acceleration reading of 9.8 m/s2 is 0.6 m/s2 and this occurs for approximately 13.0 s. Then use ? ? . 39. A Find the wavelength using ? ; ? 0.5 . Because it is initially a minimum sound intensity at point P, this means that the signals being received are 0.5 out of phase. AP=2 and BP=2.5. To receive a maximum sound intensity at point P, B needs to be moved far enough from A so that BP=3=1.5 m. Use ? ? ? to find the new distance between A and B. 40. D The mercury expands as the temperature increases. This new volume of mercury is the product of the original volume, the temperature increase, and the thermal expansion coefficient: 5.4 x 104 cm3. Using ? ? ? ? , it is found that the mercury rises 4.5 cm or 45 mm. 41. D The induced emf results from the changing area of the coil and accompanying changing flux that results from each breath and is calculated by: ? ? ? ? 42. B In an adiabatic process, no heat is allowed to flow into or out of the system, so Q=0. In an isochoric process, V is constant so Q=U, and in an isobaric process, pressure is held constant so Q=U+W. 43. A Treat the two blocks as one to find ? ? ?; Using Newtons 2nd Law in the horizontal direction for Block 1: ? ?a; ? ?g; ?a= ?; ? ? ?. Substitute for a: ? ? ?; Use ? ? 3?: ? ?