HW02-solutions

# HW02-solutions - Le(tl8426 HW02 gentle(56245 This print-out...

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Le (tl8426) – HW02 – gentle – (56245) 1 This print-out should have 31 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points Two ants race across a table 56 cm long. One travels at 4 . 4 cm / s and the other at 3 . 99999 cm / s. When the first one crosses the finish line, how far behind is the second one? Correct answer: 5 . 09104 cm. Explanation: Let : = 56 cm , v 1 = 4 . 4 cm / s , and v 2 = 3 . 99999 cm / s . The time it takes the first (faster) ant to cross the finish line is t = v 1 , and the distance the slower ant covers in that time is s 2 = v 2 t = v 2 v 1 . The slower ant is - s 2 = - v 2 v 1 = 56 cm - (3 . 99999 cm / s) (56 cm) 4 . 4 cm / s = 5 . 09104 cm from the finish line when the faster one crosses it. 002(part1of2)10.0points You drive a car 9 h at 46 km / h, then 9 h at 65 km / h. What is your average velocity? Correct answer: 55 . 5 km / h. Explanation: Let : t = 9 h , v 1 = 46 km / h , and v 2 = 65 km / h . You drive a distance Δ d = d 1 + d 2 = v 1 t + v 2 t in a time of Δ t = t 1 + t 2 = 2 t , so that the average velocity is v av = Δ d Δ t = v 1 t + v 2 t 2 t = v 1 + v 2 2 = 46 km / h + 65 km / h 2 = 55 . 5 km / h . 003(part2of2)10.0points What is your average velocity if you drive a distance of 499 . 5 km at a speed of 46 km / h, then the same distance at a speed of 65 km / h? Correct answer: 53 . 8739 km / h. Explanation: Let : d = 499 . 5 km , v 1 = 46 km / h , and v 2 = 65 km / h . Δ d = d 1 + d 2 = 2 d and Δ t = t 1 + t 2 = d v 1 + d v 2 , so that the average velocity will be v av = 2 d d v 1 + d v 2 · v 1 v 2 v 1 v 2 = 2 dv 1 v 2 dv 2 + dv 1 = 2 v 1 v 2 v 1 + v 2 = (2)(46 km / h)(65 km / h) 46 km / h + 65 km / h = 53 . 8739 km / h .

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Le (tl8426) – HW02 – gentle – (56245) 2 004 10.0points A rocket initially at rest accelerates at a rate of 60 m / s 2 for 0 . 48 min. What is its speed at the end of this time? Correct answer: 1728 m / s. Explanation: Let : a = 60 m / s 2 and t = 0 . 48 min . v f = v o + a t = a t = (60 m / s 2 ) (0 . 48 min) 60 s 1 min = 1728 m / s . 005(part1of5)10.0points Consider a toy car which can move to the right (positive direction) or left on a horizon- tal surface along a straight line. car v O + What is the acceleration-time graph if the car moves toward the right (away from the origin), speeding up at a steady rate? 1. t a 2. t a 3. t a 4. t a 5. t a 6. t a 7. None of these graphs is correct. 8. t a correct Explanation: Since the car speeds up at a steady rate, the acceleration is a constant. 006(part2of5)10.0points What is the acceleration-time graph if the car moves toward the right, slowing down at a steady rate? 1. t a
Le (tl8426) – HW02 – gentle – (56245) 3 2. t a 3. None of these graphs is correct. 4. t a 5. t a 6. t a 7. t a 8. t a correct Explanation: Since the car slows down, the acceleration is in the opposite direction. 007(part3of5)10.0points What is the acceleration-time graph if the car moves towards the left (toward the origin) at a constant velocity? 1. t a correct 2. t a 3. None of these graphs is correct. 4. t a 5. t a 6.

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