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Unformatted text preview: valdez (vv689) Homework 03 florin (58140) 1 This print-out should have 12 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points An object is shot vertically upward into the air with a positive initial velocity. What correctly describes the velocity and acceleration of the object at its maximum elevation? Velocity Acceleration 1. positive positive 2. zero zero 3. positive negative 4. negative negative 5. zero negative correct Explanation: At the maximum elevation, the vertical ve- locity is zero. The acceleration is due to gravity, which always acts down. 002 10.0 points A subway train starting from rest leaves a station with a constant acceleration. At the end of 7 . 02 s, it is moving at 20 . 0772 m / s. What is the trains displacement in the first 5 . 2299 s of motion? Correct answer: 39 . 1132 m. Explanation: Let : t = 7 . 02 s , v = 20 . 0772 m / s , and t 1 = 5 . 2299 s . a = v t , so the displacement is s = 1 2 a t 2 = 1 2 v t t 2 = 1 2 20 . 0772 m / s 7 . 02 s (5 . 2299 s) 2 = 39 . 1132 m . 003 10.0 points The acceleration of gravity is 10 m / s 2 . If there were no air resistance, with what speed would drops hit the Earth if they fell from a cloud 1705 m above the Earths sur- face? Correct answer: 184 . 662 m / s. Explanation: Drops would be in free fall and accelerate at 10 m / s 2 . We need to find the time of fall. From d = 1 2 g t 2 , the time is t = radicalBigg 2 d g = radicalBigg 2 (1705 m) 10 m / s 2 = 18 . 4662 s . gain in speed = g t = (10 m / s 2 ) (18 . 4662 s) = 184 . 662 m / s . 004 10.0 points The diagram describes the acceleration vs time behavior for a car moving in the x- direction. b b a P Q t At the point Q , the car is moving 1. with a decreasing speed. 2. with a constant speed. valdez (vv689) Homework 03 florin (58140) 2 3. with an increasing speed. correct Explanation: a = d v dt . As long as the acceleration is positive the velocity is always increasing....
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This note was uploaded on 09/26/2010 for the course PHY 58230 taught by Professor Svhets during the Spring '10 term at University of Texas at Austin.
- Spring '10