Based on the Evidence in This Video, What Are These Students Thinking About the Physics

1Department of Physics Video discussion 2Teaching Assistant Seminar 4/3/2018 1 Tutorial 2 – Green group 2 31. In a moment we’ll watch video of students working together on a mechanics tutorial. Boxed 4 below are the questions they’re discussing: work through them collaboratively with your 5 partners before we watch the video. (These questions are from the middle of the tutorial. In 6 the first part of the tutorial, students make predictions and do experiments for a number of 7 velocity and acceleration graphs.) 8 9 II. Review of basic concepts: Velocity and acceleration graphs 10A. Which feature of a velocity graph indicates your acceleration? Specifically, from the 11 velocity vs. time graph, how can you tell whether the acceleration is small or large? 12B. From the velocity vs. time graph, how can you tell whether the acceleration is 13 positive or negative? 14C. Which, if either, has more acceleration: a car cruising steadily at 60 miles per hour 15 or a rocket drifting steadily at 6000 miles per hour? Explain. 16 172. Watch the video (about 4 min). The transcript is provided below. Student 1 (S1) is in the left 18 foreground, S2 is on the left in the back, S3 is on the right in the back, and S4 is on the right 19 in the front. 20 21 Transcript: Green 2-5 22 23 24S2: Don’t they both have the same 51S3: Yeah. 25 acceleration? 52S2: Then what does your velocity graph look 26S1: Yeah, right? 53 like? 27S2: I think so. 54S3: It’s gonna be a straight line up cause you’re 28S2: Oh, wait, but… 55 cruising steadily. 29S1: If it’s cruising steadily at 60 miles, would it 56S2: It is. 30 have zero accel… 57S4: But are you increasing? 31S4: Drifting… 58S3: Yeah, so then the acceleration is a straight 32S2: Would the velocity be the same? 59 line (S2: But your velocity is not 33S2: Because even though the line’s constant, if 60 increasing, is it?) 34 it’s on a different place then there may be 61S3: No, it’s the same. 35 more of it. 62S4: See, it doesn’t say your velocity is 36S1: Yeah, right. 63 increasing, it says you’re cruising steadily. 37S1: But which, if either, has more acceleration, 64S3: So then velocity is like this, and 38 the car… if something’s going at a steady 65 acceleration is zero. 39 pace… 66S1: Yeah. 40S2: But they both are (S3: Then the 67S3: Cause you… you’re only accelerating if 41 acceleration is constant.) 68 you have your foot on the gas. 42S1: It’s not zero. 69S2: Right. 43S2: So the acceleration is constant for both of 70S3: If you like change the way that your foot is 44 them. 71 on the pedal. So acceleration is zero if it’s 45S3: No… yeah. You’re right. Wait. 72 constant velocity. 46S1: See, this new thing, yeah, it’s confusing 73S2: Right, because your velocity is like that, 47 me. 74 and the slope of that… 48S2: No, because all you have to do is look at 75S3: Is zero. 49 the velocity graph, and if you’re going 76S2: Yeah. So I guess it would be the same. 50 steadily at sixty miles an hour, right…

3 Page 1 of 4 4Department of Physics Video discussion 5Teaching Assistant Seminar 4/3/2018 77S1: Yeah, yeah. Cause the velocity graph 83S2: Yeah, the position graph would be… 78 would just be straight. 84S1: The position would be straight up. 79S2: Right, yeah. 85S2: Right. 80S1: I keep thinking that since velocity is, like, 81 displacement over time, that in the 82 distance, but… 86 87 883. Consider the following questions about the episode you just watched. 89A. These questions probably seem very basic to you, and students take quite a bit of 90 time thinking through them. What is your “gut response” as you watch them work? Do you 91 cringe at how slow they are, or are you impressed with their careful thinking (or both)? Do 92 you think they need help or are you glad no one interferes with them? What other instinctive 93 responses do you have? 94B. What are these students doing well? What tutorial goals are they attaining? 95C. Are there places where you think the students just say the wrong word, but aren’t 96 necessarily misunderstanding the physics? Point those out specifically. 97 984. The students continue with the following tutorial exercises. Do them collaboratively with 99 your partners before we watch the next video clip. The “motion detector” refers to a sonic 100 ranger that records the distance to whatever’s in front of it and displays position, velocity, 101 and/or acceleration graphs on the computer screen – ask your leader for an explanation if you 102 need one. 103 104 III. Up & down a ramp 105In this experiment, the motion detector sits at the bottom of a ramp. 106The cart starts near the bottom. You’ll give it a brief push up the ramp. 107So, after you let go, the cart rolls up the ramp, reaches its peak (highest detector 108point), and then rolls back down. The following questions refer to the 109cart’s motion after you let it go but before you catch it after it rolls back down. 110 A. Velocity Predictions: 111 (Work individually) Prediction: Sketch the velocity 112 cart’s velocity vs. time, using a dotted line. 113 (Don’t do acceleration until later.) Hint: Break 114 this into two “subproblems,” the cart’s motion 115 up the ramp, and then the motion down the time 116 ramp. 117 Compare/discuss. Graph your group consensus 118 predictions with dashed lines. peak 119A. Acceleration at the peak 120 (Work individually) Now let’s start thinking about the acceleration. At the moment the cart 121 reaches its peak, is its acceleration positive, negative, or zero? Briefly explain. Instead of 122 hashing out this issue with other students today, you’ll do so Friday in lecture. 1235. Watch the next video clip (about 5 minutes). 124

6 Page 2 of 4 7Department of Physics Video discussion 8Teaching Assistant Seminar 4/3/2018 125 Transcript: Green 2-6 126 127S1: All right, let’s start thinking about the 179 slope’s decreasing, like…) decreasing. 128 acceleration at the moment the car 180 And that’s what it should be doing. 129 reaches its peak. 181S3: Yeah. 130S3: The acceleration starts out fast, like 182S2: Cause the acceleration is decreasing, and 131 high… 183 then it holds here. 132S4: It’s gonna be going from positive to 184S3: Yeah. 133 negative 185S4: And then it slopes down. 134S1: So it’s zero, (with S3: zero at the peak). 186S1: Oh no, we’re running out of time. 135 Yeah. 187S3: Let’s stick with it. 136S3: That we know. 188S1: Yeah. 137S2: Right, because the slope… (?) 189TA: So you guys… 138S2: Yeah, we figured it out. 190S2: Yeah, we’re sticking with it. 139S1: We fixed it. 191TA: What was that? 140TA: What does it look like? Hm. 192S2: We’re sticking with it. 141S1: Cause it’s going the opposite direction, 193TA: You still discussing? 142 so thus it would have a negative velocity. 194S1: We think it’s… 143TA: I see. 195S2: We think we’re right. 144S1: We’re guessing. 196TA: What’s your… what’s the reasoning? 145TA: Do you guys agree that it’s curved like 197S1: Cause… well, it’s decreasing, but it’s 146 that? 198 going the right direction, so it’s… well, 147S2: (laughs) We did. 199 basically (?), the way down, it’s easier. 148S4: (?) 200 We know it’s speeding up, just cause 149TA: I’ll let you guys discuss. That’s an 201 gravity, so like the slope is increasing, 150 interesting question to consider. 202 like if it was… you know what I mean? 151S2: Torture. This is torture. 203TA: Right, but let me ask you this. It was 152S4: Where’s that other guy? 204 speeding up in the other graphs you 153S3: Wait, let’s just think about this. 205 drew, did you have it curved in the other 154S2: Yeah, let’s. 206 graphs? 155S4: It starts out fast, and then goes slow. 207S1: No, it was straight, that’s right. 156S2: But it holds, it holds… (S4: at first the 208S2: But we thought that the acceleration was 157 accel…) at the peak. 209 constant for the other ones, and for this 158S3: Yeah, that part’s right. 210 one that it’s not, maybe. 159S2: So it can’t just be like that (S3: Oh, 211TA: Well, let me ask you this. What causes 160 nonono) unless it goes like that. 212 the acceleration? 161S3: I think it’s whether or not it’s (S1: It’s 213S3: Gravity going down. 162 just a straight line?) like this or like this. 214TA: Why is the acceleration constant, that’s 163S1: Oh. 215 the big question. 164S3: It’s definitely gonna curve. 216S3: Because gravity’s constant? 165S4: Well you can see it goes up and down. 217TA: I don’t know, you’re supposed to tell 166S1: Yeah. 218 me. 167S2: In fact, there is acceleration, so it can’t 219S3: Well, obviously gravity doesn’t change. 168 be just a straight line. 220TA: OK. 169S3: Right, but… 221S3: So it should be a straight line. 170S1: I think it (S3: Which way does it curve?) 222TA: I don’t know. 171 would have to be how we had it, cause 223S1: I guess. 172 like, the slope of it is getting steeper 224S3: Yeah, because the initial force… like, 173 there, which means it’s like… you know 225 there’s no… once we push it, there’s no 174 what I mean? 226 other forces acting on it to make it (S1: 175S3: It’s getting… I think the bottom part is 227 Yeah) you know? Except for the fact 176 right. 228 that it just slows down because of gravity 177S2: Speeding up towards the end. Right, so 229 (S1: the ramp). 178 this part the slope is (S1: This too, the 230S3: And when it rolls back down, there’s 231 nothing else acting on it except for

9 Page 3 of 4 10Department of Physics Video discussion 11Teaching Assistant Seminar 4/3/2018 232 gravity, which doesn’t change either. So 247S2: Ohh. 233 it is a straight line. (laughs) 248S1: If you figure, we were always thinking 234S1: That makes sense. 249 this is curved, but maybe that’s just kind 235S2: But what about the period of rest when it 250 of our push, and it like starts just there, 236 brakes? 251 but it’s so small that like (S4: you can’t 237S3: Then that’s (S1: that’s zero) where it hits 252 see it) it’s really just like a curved line 238 zero at the peak. Velocity is zero. 253 and you can’t see it. 239S1: That’s (S3: the acceleration is still the 254TA: Is that the graph? 240 same) where the force of gravity cancels 255S2: Yeah, see why we’re all messed up? 241 out the… our pushing force. 256S3: The acceleration… the acceleration is… 242S2: But it sat there for a long time and we’re 257TA: The acceleration looks like zero almost. 243 just making it one little point. 258 Oh, OK. Let’s try graphing that again, 244TA: Did it sit there for a time or was it just 259 let’s try actually getting a measurement. 245 that it was slowing down the one way 246 and speeding up the other way? 260 261 2626. Consider the following questions about the episode you just watched. 263A. At the beginning of the clip, the students are using the word “acceleration,” but they 264 might mean “velocity.” What do you think? 265B. When the TA says “Do you guys agree that it’s curved like that?” the briefly- 266 confident students are tipped off that their answer is wrong. What effect does that have on 267 them? 268C. Why does the TA leave just after asking about the curve? How do the students 269 respond to his leaving? 270D. At one point student 1 suggests a straight line for the velocity graph, but students 2 271 and 3 are convinced the line is curved: student 3 says “I think it’s whether or not it’s like this 272 or like this,” demonstrating two different curvatures, and student 2 says “In fact, there is 273 acceleration, so it can’t be just a straight line.” What do you think they are thinking? 274E. What curvature do they decide on? In what sense is their graph correct? 275F. By the end of the clip, students seem to have decided on the correct graph. What do 276 they say about why this graph seemed to them to be different from the previous ones (curved 277 instead of straight)? Do you believe them? (The suggestion is not that they’re being 278 intentionally misleading – just that they might not know their own minds.) 279

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