Ch2-HW3-solutions

Ch2-HW3-solutions - schopper(as55864 – Ch2-HW3 – haley...

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Unformatted text preview: schopper (as55864) – Ch2-HW3 – haley – (56465) 1 This print-out should have 22 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 (part 1 of 4) 2.5 points A runner starts from rest and in 3 s reaches a speed of 8 m / s. If we assume that the speed changed at a constant rate (constant net force), what was the average speed during this 3 s interval? Answer in m/s. Correct answer: 4 m / s. Explanation: Let’s assume motion in the + x direction. Since the speed changed at a constant rate, the average speed is equal to the arithmetic mean of the initial and final speeds: v avg ,x = v i,x + v f,x 2 = 0 + 8 m / s 2 = 4 m / s . 002 (part 2 of 4) 2.5 points How far did the runner go in this 3 s interval? Answer in m. Correct answer: 12 m. Explanation: Since the speed increased at a constant rate, we can just treat the situation as though the runner were running at the average velocity the whole time: Δ x = v avg ,x Δ t = (4 m / s)(3 s) = 12 m . 003 (part 3 of 4) 2.5 points The driver of a car traveling at a speed of 18 m / s slams on the brakes and comes to a stop in 4 s. If we assume that the speed changed at a constant rate (constant net force), what was the average speed during this 4 s interval? Answer in m/s. Correct answer: 9 m / s. Explanation: We follow the same steps from part 1: v avg ,x = v i,x + v f,x 2 = 18 m / s + 0 2 = 9 m / s . 004 (part 4 of 4) 2.5 points How far did the car go in this 4 s interval? Answer in m. Correct answer: 36 m. Explanation: We follow the same steps from part 2: Δ x = v avg ,x Δ t = (9 m / s)(4 s) = 36 m . 005 (part 1 of 3) 4.0 points On a straight road (taken to be in the + x direction) you drive for 3 hours at a constant v 1 ,x = 30 mph , then in a few seconds you speed up to v 2 ,x = 65 mph and drive for an additional hour. What is the x component of average ve- locity ( v avg ,x ) for the 4-hour period, using the fundamental definition of average veloc- ity, which is the displacement divided by the time interval? Answer in mph. Correct answer: 38 . 75 mph. Explanation: schopper (as55864) – Ch2-HW3 – haley – (56465) 2 To find the x component of average velocity, we have to divide the total displacement by the full time interval: v avg ,x = v 1 ,x t 1 + v 2 ,x t 2 t 1 + t 2 = (30 mph)(3 h) + (65 mph)(1 h) 3 h + 1 h = 38 . 75 mph . 006 (part 2 of 3) 3.0 points Suppose that instead you use the formula v * avg ,x = v i,x + v f,x 2 . What do you calculate for the x component of average velocity now? Answer in mph....
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This note was uploaded on 09/12/2011 for the course PHY 303K taught by Professor Turner during the Fall '08 term at University of Texas.

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Ch2-HW3-solutions - schopper(as55864 – Ch2-HW3 – haley...

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