Brian Shotwell, Department of Physics University of California, San Diego Physics 2C (Fluids/Waves/Thermo/Optics), Spring 2019 PRACTICE QUIZ 1

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1 Brian Shotwell, Department of Physics University of California, San Diego Physics 2C (Fluids/Waves/Thermo/Optics), Spring 2019 PRACTICE QUIZ 1 All students must work independently. You are allowed one page of handwritten notes only; no communication devices (cell phones, etc) permitted. Show all work; no credit will be given for answers with no derivation. Any problem asking for a vector (e.g., force) requires a vector as an answer! Directions: Work each problem on the exam sheet provided. Put your NAME and PID on EACH sheet (one for each problem). If you don t have enough room on a single side of the page, finish the problem on the back of the page = 55 points total for this exam. Page 0 of 5

2 1. (8 points, 2 points each): Label the following statements as True or False. Provide a (at least) one-sentence explanation for each; correct answers without an explanation receive no credit. (a) A one square meter column of air extending from sea level to space has more mass (from the air alone) than the mass of a car (assume a car is about 1000 kg). (b) If an ice-cube floating in a glass of water melts, then the water level rises. (c) In class, the quietest we got the class was 41 db and the loudest was 99 db. This corresponds to a change of absolute intensity of more than 10 5 W/m 2. (d) Suppose you double your distance from an isotropic EM-wave source (like a light bulb, emitting equally in all directions in 3D). As a result of the move, the amplitude of EM waves decreases by a factor of two. Page 1 of 5

3 2. (12 points): Identically-shaped lead and aluminum cubes are suspended at different depths by two wires in a large vat of water: (a) Which cube experiences a greater buoyant force? (2 points) (b) For which cube is the tension in the wire greater? (2 points) (c) Which cube experiences a greater force on its lower face? (2 points) (d) If the density of lead is ρ lead and the tension in the cord connected to the lead box is T, what is the volume of each box? (6 points) Page 2 of 5

4 3. (10 points, 5 points each): Water is flowing in a pipe with a circular cross section but with varying cross-sectional area, and at all points the water completely fills the pipe. Water is flowing into this pipe at a steady rate of 1.20 m 3 /s. (a) At one point in the pipe the radius is m. What is the speed of the water at this point in the pipe? (5 points) (b) Suppose the radius doubles as compared to the point in part (a), to a new radius of m. If there is no appreciable change in vertical height, what is the corresponding change in pressure? (5 points) Page 3 of 5

5 4. (15 points, 5 points each): A sinusoidal wave is propagating along a stretched string that lies along the x-axis. The displacement of the string as a function of time is graphed below for particles at x = 0 and at x = m. (a) What is the amplitude of the wave? What is the period of the wave? (b) You are told that the two points x = 0 and x = m are within one wavelength of each other. If the wave is moving in the ˆx direction, determine the wavelength and the wave speed. (c) Give an equation y(x, t) for this wave valid for all x and all t. Have your equation be of the form y(x, t) = A sin(kx ± ωt + φ 0 ), where the phase constant φ 0 [0, 2π). Page 4 of 5

6 5. (10 points, 5 points each): A loudspeaker of power 2.0 W emits sound waves of frequency Hz spread out equally in one hemisphere. That is, the source is not exactly isotropic, since the wave energy is spread out over one hemisphere rather than an entire sphere. (a) What is absolute intensity a distance 25 meters from the loudspeaker? What is the intensity in db a distance 25 meters from the loudspeaker? (b) Someone really hates Hz but likes Hz. What is the minimum speed the person must run to hear this frequency? Page 5 of 5