Physics 7Em Midterm Exam 1

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1 Physics 7Em Midterm Exam 1 MULTIPLE CHOICE PROBLEMS. There are 10 multiple choice problems. Each is worth 2 points. There is no penalty for wrong answers. In each, choose the best answer; only one answer is correct. Be sure to transfer your answers to the front of the exam. 1. A loudspeaker emits sound waves equally in all directions. A person 10 m away would like to move closer so that the sound level is raised by 20 db. How far away should the person be from the loudspeaker? (A) 1.0 m. (B) 2.5 m. (C) 3.3 m. (D) 5.0 m. (E) There is not enough information to determine the answer. 2. A thick string with linear density µ is joined to a thin string with linear density 1 µ. An incident 2 pulse is sent down the thick string toward the thin string and eventually creates reflected and transmitted pulses. Which of the following is true? (A) The reflected and transmitted pulses are both not inverted. (B) The reflected and transmitted pulses are both inverted. (C) The reflected pulse is inverted, but the transmitted pulse isn t. (D) The transmitted pulse is inverted, but the reflected pulse isn t. (E) There is not enough information to determine the answer. 3. A pendulum made from a mass on the end of a string oscillates in simple harmonic motion. To increase the pendulums period of oscillation, one could (A) Increase the mass at the end of the pendulum. (B) Move it to a higher altitude. (C) Shorten the string. (D) Two or more of the above would increase the period. (E) None of the above would increase the period. 4. An ice cube (made of water with density of kg/m 3 ) sits in a cup of sea water (density kg/m 3 ). Neglecting surface tension, what happens to the level of the water as the ice cube melts? (A) The water level remains unchanged. (B) The water level drops.

2 Physics 7Em Midterm Exam 2 (C) The water level rises. (D) The water level first rises and then drops back to its initial position. (E) The water level first drops and then rises back to its initial position. 5. A piston container is filled with air and sealed. The piston moves with sinusoidal oscillations with frequency f, creating sound waves of speed v with wavelength λ. The pistons oscillation frequency is then increased to 2f. What is the speed and the wavelength of the resulting sound waves? (A) v and 1 2 λ. (B) v and λ. (C) 2v and 1 2 λ. (D) 2v and λ. (E) There is not enough information to determine the answer. 6. An aquarium tank is in the shape of a cylinder, with 6 meter tall glass wall. When the tank is filled with water that is 2 meter deep, the water exerts a force F on the wall. What force does the water exert on the wall when the tank is filled with 4 meter deep water? (A) 4F (B) 2F (C) F (D) 1 2 F (E) There is not enough information to determine the answer. 7. A wave has wave function y(x, t) = A sin(kx + ωt). Let v be the wave velocity and v y be the transverse velocity of the wave at x = 0. Which of the following is true? (A) v = ω k and v y = Aω cosωt (B) v = ω k and v y = Aω cos ωt (C) v = ω k and v y = Aω cosωt (D) v = ω k and v y = Aω cos ωt (E) None of the above is true. 8. A mass on a spring is displaced 1.0 m and let go from rest. It then moves in simple harmonic motion with maximal acceleration 1.0 m/s 2. How long does it take to complete one cycle? (A) 0.25 s.

3 Physics 7Em Midterm Exam 3 (B) 0.50 s. (C) 1.00 s. (D) 3.14 s. (E) 6.28 s. 9. Water flows upward in a pipe with constant cross sectional area. The difference between the water pressure at ground level and the water pressure at a point 20 meters above ground level is (A) 49 kpa (B) 98 kpa. (C) 196 kpa. (D) 392 kpa. (E) None of the above; it depends on the velocity of the water. 10. A very long drinking straw collapses when the difference between the outside pressure and the inside pressure exceeds 0.4 atm. A person on the surface of the Earth sucks water up the straw. What is the maximum distance the water can be raised before the straw collapses? (A) 0 m. (B) 4.12 m. (C) 6.18 m. (D) 10.3 m. (E) 41.2 m.

4 Physics 7Em Midterm Exam 4 FREE RESPONSE QUESTIONS. There are five free response problems, each worth either 8 points. Do all problems and show all work for full credit. 1. After graduating, a student goes to work for a company that makes golf balls. Her first assignment is to design a golf ball with mass m = kg that floats in water and is made of a material with uniform density ρ s = 1500kg/m 3. The golf ball is spherical with an empty spherical cavity. If half of the ball is floating above the water, (A) what is the outer radius of the ball? (4 pts) (B) what is the radius of the empty cavity? (4 pts)

5 Physics 7Em Midterm Exam 5 2. A stationary police car emits 1000 Hz sound waves. A truck driver passes the police car and drives away at 80 miles per hour, or roughly 36 m/s. The temperature is 20 o C and there is no wind. (A) What is the frequency of the sound heard by the truck driver? (4 pts) (B) The sound is now reflected by the truck. What is the frequency of the reflected sound heard by the police officer sitting in the police car? (4 pts)

6 Physics 7Em Midterm Exam 6 3. A glider with mass m = 1.00 kg is attached to a spring and moves without friction on an air track. The glider is moved a distance d = m from its equilibrium position and released from rest. The next time the speed of the glider is zero is t = s later. (A) What is the spring constant of the spring? (4 pts) (B) What is the maximum acceleration of the glider? (2 pts) (C) A damping force is added to make the glider-spring system critically damped. What is the coefficient of damping b? (2 pts)

7 Physics 7Em Midterm Exam 7 4. At a concert, the sound from a loudspeaker that is 10 m away reaches your ears with sound level 50 db. (8 pts) (A) Assume your eardrums each have area A = m 2. How much sound power is incident on each eardrum? (3 pts) (B) What s the pressure amplitude P max associated with this sound level? (Assume the air temperature is 20 o C.) (3 pts) (C) How many loudspeakers are needed if we want to increase the sound level to 60 db? (2 pts)

8 Physics 7Em Midterm Exam 8 5. Derive the fact that a disturbance on a string with tension T and linear mass density µ is governed by the linear wave equation with wave velocity v = T/µ. Carefully note any approximations that are used in the derivation. (8 pts)

Physics 5B PRACTICE MIDTERM EXAM I-B Winter 2009

Physics 5B PRACTICE MIDTERM EXAM I-B Winter 2009 PART I: Multiple choice questions Only one of the choices given is the correct answer. No explanation for your choice is required. Each multiple choice