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PH 211 Winter 2014 Wednesday January 15 Homework 2 hand-in There is one problem for set 2 (Chapter 1) to hand in. Because of the nature of the problems, I cannot use the assigned problems in Mastering Physics, so I assign problem 1.42 from the end of the chapter. Hand in your written homework to box 12 on the second floor, before Friday 5pm. Make sure you write your name clearly on the top of the sheet! This first problem is not meant to be difficult, my goal is to get everybody, including the grader and myself, in the routine! For this problem, draw a complete pictorial representation! Do not use any mathematics to get the answer! Ice hockey star Bruce Blades is 5.0 m from the blue line and gliding toward it at a speed of 4.0 m/s. You are 20 m from the blue line, directly behind Bruce. You want to pass the puck to Bruce. With what speed should you shoot the puck down the ice so that it reaches Bruce exactly as he crosses the blue line? Google moderator Which topic would you like to discuss in class on Friday? http://www.google.com/moderator/#15/e=211 071&t=211071.40 Google moderator Today’s questions: http://www.google.com/moderator/#16/e=20e 0eb Which of these plots shows a positive velocity and a negative acceleration? x A B x t x C t x t D t Which of these plots shows a positive velocity and a negative acceleration? A B C D 86% 2% D C 6% B 6% A A. B. C. D. In this x(t) plot at how many points in time is the acceleration equal to zero? In this x(t) plot at how many points in time is the acceleration equal to zero? A. B. C. D. E. 0 2 4 6 8 84% 5% 0 2% 2 4 5% 4% 6 8 In this x(t) plot at how many points in time is the acceleration equal to zero? Velocity large, positive x Velocity small, positive x t t x t x x Velocity zero t Velocity large, negative t Velocity small, negative Practice: draw v(t) and a(t) for this graph x A B C D E t Moe stands on a cliff with two balls. One is thrown straight up with initial speed vo, the other is thrown straight down with the same initial speed, vo. How do the final velocities of the two balls compare right before they hit the ground? DRAW a motion diagram OR graph to JUSTIFY your choice! (what assumptions are you making/model are you using) 1. The ball thrown up is faster right before it hits the ground 2. The ball thrown down is faster right before it hits the ground 3. The two balls have the same speed right before they hit the ground 4. There isn’t enough information to determine the answer based on what is given Moe stands on a cliff with two balls. One is thrown straight up with initial speed vo, the other is thrown straight down with the same initial speed, vo. How do the final velocities of the two balls compare right before they hit the ground? A. 67% ... h no ug ’t e isn er e Th in fo r sa .. th e ve ba lls o w et Th 10% ha n w hr o lt al eb Th eb al lt hr o w n up do w is n is fa s ... t. . 12% 10% Th The ball thrown up is faster right before it hits the ground B. The ball thrown down is faster right before it hits the ground C. The two balls have the same speed right before they hit the ground D. There isn’t enough information to determine the answer based on what is given Asked to graph the velocity vs. time of a certain motion, a researcher draws graph 1 below. Then he draws graph 2, showing the position vs. time for the same motion. • The researcher who drew the above graphs is 99% sure he drew graph 2 correctly. Is there some way he can use his position graph to check for inconsistencies with his velocity graph? If so, do it and explain your reasoning. • If asked to draw a velocity graph for motion someone just observed, why might they first draw the position graph? (most experienced people do this) The graph shows position as a function of time for two trains running on parallel tracks. Which is true: 1. At time tB, both trains have the same velocity. 2. Both trains speed up all the time. 3. Both trains have the same velocity at some time before tB. 4. Somewhere on the graph, both trains have the same acceleration The graph shows position as a function of time for two trains running on parallel tracks. Which is true: A. At time tB, both trains have the same velocity. B. Both trains speed up all the time. C. Both trains have the same velocity at some time before tB. D. Somewhere on the graph, both trains have the same acceleration 88% 8% gr a sa th e th e on av e So m ew he re sh ain tr th Bo ph .. . m l. al up ed ss pe ain tr th Bo ... .. . sh a. ain tr ot h ,b tB e tim At 4% 0% Draw x(t) for a case where the position and acceleration always have opposite signs. Average velocity. − = − I drive from my house to another city 60 km away in one hour, spend one hour having lunch in that city, and drive back in one hour. How large is the average velocity for the trip? A.0 km/hour B.20 km/hour C.40 km/hour D.60 km/hour I drive from my house to another city 60 km away in one hour, spend one hour having lunch in that city, and drive back in one hour. How large is the average velocity for the trip? 0 km/hour 20 km/hour 40 km/hour 60 km/hour 38% ur /h o km 60 40 km /h o ur 7% ur /h o km 20 km /h ou r 8% 0 A. B. C. D. 47% I drive from my house to another city 60 km away in one hour, spend one hour having lunch in that city, and drive back in one hour. How large is the average speed for the trip? A.0 km/hour B.20 km/hour C.40 km/hour D.60 km/hour I drive from my house to another city 60 km away in one hour, spend one hour having lunch in that city, and drive back in one hour. How large is the average speed for the trip? 79% 0 km/hour 20 km/hour 40 km/hour 60 km/hour 13% 5% ur 60 km /h o ur 40 km /h o ur km /h o 20 km /h ou r 3% 0 A. B. C. D. Average speed: Total DISTANCE travelled divided by total time! Instantaneous velocity. + ∆ − = = ∆ Instantaneous velocity. = + + () = + + () Instantaneous velocity, for computational folks. = + 1 ∆ 2 − − ∆ 1 ∆ 2 =