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Unformatted text preview: reyes (mr29667) – Homework 2 – chelikowsky – (59005) 1 This printout should have 19 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Two ants race across a table 50 cm long. One travels at 4 . 37 cm / s and the other at 2 cm / s. When the first one crosses the finish line, how far behind is the second one? Correct answer: 27 . 1167 cm. Explanation: Let : ℓ = 50 cm , v 1 = 4 . 37 cm / s , and v 2 = 2 cm / s . You first have to compute the time it takes the first (faster) ant to cross the finish line: t = ℓ v 1 , then you must compute the distance the slower ant covers in that time: s 2 = v 2 t = v 2 ℓ v 1 . The slower ant is ℓ − s 2 = ℓ − v 2 ℓ v 1 = (50 cm) − (2 cm / s) (50 cm) (4 . 37 cm / s) = 27 . 1167 cm from the finish line when the faster one crosses it. 002 10.0 points Assume: 1 . 37 km / lap. A race car is one lap behind the lead race car when the lead car has 49 laps to go in a race. If the speed of the lead car is 52 . 4 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: 53 . 4694 m / s. Explanation: Basic Concepts Constant Velocity Solution The time needed for the lead car to end the race is t = n v = (49 laps) (52 . 4 m / s) = (49 laps) (1 . 37 km / lap) (52 . 4 m / s) parenleftbigg 1000 m 1 km parenrightbigg = 1281 . 11 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 = (50 laps) (1281 . 11 s) = (50 laps) (1 . 37 km / lap) (1281 . 11 s) parenleftbigg 1000 m 1 km parenrightbigg = 53 . 4694 m / s . 003 10.0 points What is the acceleration of a vehicle that changes its velocity from 200 km / h to a dead stop in 50 s ? Correct answer: − 1 . 11111 m / s 2 . Explanation: The acceleration is a = v t = − 200 km / h 50 s · 1000 m 1 km · 1 h 60 min · 1 min 60 s = − 1 . 11111 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 . reyes (mr29667) – 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. None of these graphs are correct. 4. t a t t 1 5. t a t t 1 6. t a t t 1 7. t a t t 1 8. t a t t 1 correct 9. t a t t 1 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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This note was uploaded on 07/24/2010 for the course PHY 59005 taught by Professor Chelikowsky during the Fall '09 term at University of Texas.
 Fall '09
 Chelikowsky

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