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Unformatted text preview: yindeemark (rry82) Homework 2 chelikowsky (59005) 1 This print-out should have 19 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points Two ants race across a table 68 cm long. One travels at 4 . 04 cm / s and the other at 3 cm / s. When the first one crosses the finish line, how far behind is the second one? Correct answer: 17 . 505 cm. Explanation: Let : = 68 cm , v 1 = 4 . 04 cm / s , and v 2 = 3 cm / s . You first have to compute the time it takes the first (faster) ant to cross the finish line: t = v 1 , then you must compute the distance the slower ant covers in that time: s 2 = v 2 t = v 2 v 1 . The slower ant is s 2 = v 2 v 1 = (68 cm) (3 cm / s) (68 cm) (4 . 04 cm / s) = 17 . 505 cm from the finish line when the faster one crosses it. 002 10.0 points Assume: 1 . 74 km / lap. A race car is one lap behind the lead race car when the lead car has 60 laps to go in a race. If the speed of the lead car is 69 . 6 m / s, what must be the average speed of the second car to catch the lead car just before the end of the race ( i. e., right at the finish line)? Correct answer: 70 . 76 m / s. Explanation: Basic Concepts Constant Velocity Solution The time needed for the lead car to end the race is t = n v = (60 laps) (69 . 6 m / s) = (60 laps) (1 . 74 km / lap) (69 . 6 m / s) parenleftbigg 1000 m 1 km parenrightbigg = 1500 s . In the same period of time, the trailing car must go 1 lap more than the lead car to catch up; thus its average speed must be v avg = m t = (61 laps) (1500 s) = (61 laps) (1 . 74 km / lap) (1500 s) parenleftbigg 1000 m 1 km parenrightbigg = 70 . 76 m / s . 003 10.0 points What is the acceleration of a vehicle that changes its velocity from 500 km / h to a dead stop in 30 s ? Correct answer: 4 . 62963 m / s 2 . Explanation: The acceleration is a = v t = 500 km / h 30 s 1000 m 1 km 1 h 60 min 1 min 60 s = 4 . 62963 m / s 2 . 004 10.0 points Henry hits a hockey puck at time, t t , which is stopped by a net starting at time, t t 1 . yindeemark (rry82) Homework 2 chelikowsky (59005) 2 Which of the following curves could de- scribe the acceleration of the hockey puck? 1. t a t t 1 2. t a t t 1 3. t a t t 1 4. t a t t 1 5. t a t t 1 6. None of these graphs are correct. 7. t a t t 1 8. t a t t 1 9. t a t t 1 correct Explanation: At the moment Henry hits the hockey puck, it gains the maximum acceleration, the ve- locity increases quickly to a constant value, which remains unchanged (with a zero accel- eration) until it gets stopped by the net at time t 1 ....
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- Fall '08