CT2-1 A person starts in Boulder, drives to Denver, 50 km away, in 1 hour, stays in Denver for 1 hour, then speeds back to Boulder in 30 minutes. What is the average speed of this round trip?A: zero B: 67 km/hr C: 40 km/hrD: 75 km/hr E: None of these.What is the average velocity of this round trip? (same choices)Answers: Average speed is (100 km) / (2.5 hr) = 40 km/hrAverage velocity = 0 sincexvtD=D and xfinal = xinitial  v = 0 CT2-2A person initially at point3 on the x-axis stays there for alittle while and then strolls alongthe x-axis to point 1, stays therefor a moment and then runs topoint 2 and remains there. Which of the following graphscorrectly depicts this motion?startfinishDenver50 km in 1 hour50 km in 1/2 hour(Wait 1 hour).0 1 2 3x3 1 2slow fastAnswer: DtimextimextimextimexA BCDE: None of these!CT2-3 A object starts at x = +5 and moves left along the x-axis at constant speed:Which graph represents this motion?Answer: B0xfinishstarttimextimextimextimexA BCDE: None of these!CT2-4A train car moves along a long straight track. The graph shows the position as a function of time for the train. The graph shows that the train:A: speeds up all the time.B: slows down all the time.C: speeds up part of the time and slows down part of the time.D: moves at constant velocity.E: None of these statements is true.Answer: B. The train is slowing down all the time because the slope of the curve is always decreasing. On a graph of x vs. t , the slope is the velocity.timepositionCT2-5 A train moves along a straight track and its position vs. time looks like:Which graph best depicts the train's velocity vs. time?Answer: DtxtvvAvCvDBtttCT2-6 The graph show positionsas a function of time for two trainsrunning on parallel tracks.Which statement is true:A: At time tC, both trains have thesame velocity.B: Both trains speed up all thetime.C: Both trains have the same velocity at some time before tC.D: At some time, both trains have the same acceleration.E: None of the above statements is true. Answer: B: Both trains have the same velocity when the slopes of the two curves are equal. Notice that train A has constant velocity and therefore has zero acceleration. Train B is slowing down all the time. Train B has negative velocity.timepositiontrain Atrain BtCCT2-7 An object's velocity vs. time is:Which graph best represents the object's acceleration vs. time?Answer: D. The slope of a curve of v vs. t is the acceleration a.taAtaBtaCtaDtvCT2-8 At the moment that the velocity is zero, what is the sign of the acceleration?A) Positive B) negative C) zeroIs the acceleration constant all during the time the velocity is changing?A) Yes, a = constant B) No, a is changing.Answers: A) the acceleration is negative at the moment when the velocity is zero. A) the acceleration is constant. The slope of a graph of v vs. t is the acceleration. Although v = 0 at an instant of time, the velocity is in the process of changing from positive v to negative v.vta = ?CT2-9A glider on a tilted air track is given a brief push uphill. The glider coasts up to near the top end, stops, and then slides back down. When the glider is at the highest point of its path, its acceleration is..A: straight down B: downward along the trackC: upward along the track D: no direction, the acceleration is zero.Answer: B. the direction of "Delta-v" is downward along the track. Just before the glider reaches the top, the velocity v1 is upward along the track. Just after the glider reaches the top, the velocity v2 is downward along the track. The change in velocity v is the vector that changes v1 into v2. The direction of the acceleration is the same as the direction of