Based on the Evidence in This Video, What Are These Students Thinking About the Physics s1

1Department of Physics Video discussion 2Teaching Assistant Seminar 4/26/2018 1 Tutorial 3 – 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. 6 7Previous tutorials and Interactive Lecture Demonstrations introduced strategies for reconciling 8common sense with physics concepts when they seem to contradict each other. You’ll practice 9those strategies here.

10 I. “Timmy’s fallen down the well!” 11To rescue a child who has fallen down a well, rescue workers fasten him to a rope, the other end 12of which is then reeled in by a machine. The rope pulls the child straight upward at steady speed. 13The child weighs 250 newtons, which means gravity pulls him downward with 250 newtons of 14force. 15A. (Work together) Draw a diagram of this situation that you can refer to during subsequent 16 discussions. 17B. (Work individually) As the child is pulled upward at constant speed, does the rope exert an 18 upward force greater than, less than, or equal to 250 newtons? Explain. If you have 19 competing arguments, give them both! 20C. (Work together) If you didn’t do so in part B, give an intuitive argument that the rope exerts 21 a force greater than 250 newtons. 22D. (Work together) If you didn’t do so in part B, use Newton’s second law to determine 23 whether the rope exerts a force greater than, less than, or equal to 250 newtons. (Hint: The 24 rope pulls the child with constant velocity. So what’s the acceleration?) 25E. (Work together) Are you 100% comfortable with your understanding of this scenario, or is 26 there still something that needs to be reconciled? Explain. 27 28 292. Watch the video (about 2 min). The transcript is provided below. Student 1 (S1) is in the left 30 foreground, S2 is on the left in the back, S3 is on the right in the back, and S4 is on the right 31 in the front. 32 Transcript: Green 3-1 33 34S2: OK, so, an object acted on by another... 48TA: So what's going on, what have you got 35 object changes (S4: Isn't it F=ma?) its 49 here? 36 velocity according to the rule a equals Fnet 50S3: Well, are we, are we supposed to be talking 37 over m. 51 about the time when he goes to rest... from 38S4: Could it mean(?) force equals mass times 52 rest to movement, or just when he's 39 acceleration 53 moving? 40S3: So, his acceleration is zero, so that means 54TA: This question... the next question is gonna 41 the force has to be 250 Newtons. 55 be what happens to go from rest to 42S2: So it's equal? 56 movement. 43S4: Ah, yeah, so I guess they do have the same 57S3: Oh. 44 acceleration, which would make sense. 58TA: But this one only applies to when he's 45S3: He has no acceleration, he's going the same 59 moving. 46 velocity. 60S3: Oh. 47S4: So maybe the forces are different.

3 Page 1 of 5 4Department of Physics Video discussion 5Teaching Assistant Seminar 4/26/2018 61TA: But your reasoning, if you're asking that 87 he's moving, right? Or am I just totally 62 question, is probably on the right track. 88 wrong? 63S3: OK. 89S2: Yeah, cause if he's moving... 64TA: Um, but this is at the point where he's just 90S3: Well, going from rest, he's accelerating. 65 moving. 91S2: Right, when he's moving, the acceleration's 66S3: Oh. 92 just constant. 67S1: So why... 93S3: It's zero. 68S4: But either way, wouldn't the accelerations 94S2: Cause he's moving. 69 be the same? 95S4: Yeah. 70S3: No, when you're moving at a constant rate, 96S2: Right. 71 you're not accelerating. 97S4: But say, when he's sitting, and then he 72S4: Yeah, but like, say, if he's from rest and 98 starts, that's an acceleration. (S2/S3: right, 73 then he moved, the rope's gonna be at rest 99 that's acceleration, right). But the rope, 74 and then it's gonna start moving... with the 100 when he's just sitting there, say it's like a 75 machine... 101 machine that twirls a rope around... 76S3: Are you saying the acceleration of the rope 102S2: Mmmhmm. 77 and him are different? 103S4: That'll be stopped, and then it's gonna 78S4: No, I'm saying they're the same. 104 move too, so its change should be like, 79S3: I don't think, yeah, I don't think... 105 proportional. 80S2: Right, yeah. 106S3: Yeah, I think... 81S3: Yeah, they're the same. 107S4: Or am I just an idiot? 82S2: Yeah. 108S3: They're the same. 83S4: Yeah. 109S4: Yeah. So either way, like, the force... is 84S2: Yeah. 110 different? I don't know. We'll figure it out. 85S4: So it really wouldn't make a difference if 86 he's starting from rest or if he's starting, if 111 112 1133. Consider the following questions about the episode you just watched. 114 A. What do you think S4 is thinking when he says, “I guess they do have the same 115 acceleration, which would make sense”? What about just after that when he says to the 116 TA, “So maybe the forces are different”? 117 B. Why does S3 ask the TA whether they’re talking about the start of the motion, or just the 118 constant-velocity part? (What inspires her to ask that?) 119 C. The TA says “Your reasoning, if you’re asking that question, is probably on the right 120 track.” What do you think? Is the group on track, or just S3, or not necessarily either 121 one? What is the possible benefit or detriment of the TA saying that? 122 D. In the second half of the clip, the TA says very little, although he’s still there and 123 listening. Why do you think he does that? What message do you think that sends to the 124 students? 125 E. As the clip ends, it seems that neither the TA nor the rest of the group understands what 126 S4 is talking about. If you were the TA, what would you do at this moment? 127 F. Suppose you knew from other interactions that S3 was an extremely strong student. How 128 would that affect your interactions with this group? 129 1304. The students continue with the tutorial exercises on the following page. Do them 131 collaboratively with your partners before we watch the next video clip. 132

6 Page 2 of 5 7Department of Physics Video discussion 8Teaching Assistant Seminar 4/26/2018 133II. Refining intuition to reconcile Newton’s laws with common sense 134Most students have, or can at least sympathize with, the intuition that upward motion requires an 135upward force, in which case the upward rope force must “beat” the downward gravitational force 136to make the child move up. Can we reconcile that intuition with the Newtonian conclusion that 137the upward force merely equals the downward force? 138In a previous tutorial and in lecture, you learned about Refining intuition as a reconciliation 139strategy. That’s how we reconciled Newton’s third law with the intuition that a lighter object 140reacts more during a collision. Let’s see if refining intuition works here. 141A. (Work together) Consider the child, initially at rest, right when the rope first starts to pull 142 him upward. During that initiation stage of the motion, is the upward force from the rope 143 greater than, less than, or equal to 250 newtons (the child’s weight)? 144 1. What does Newton’s second law say about this question? (Hint: Is the child accelerating 145 during the initiation of the motion?) 146 2. Does the Newtonian answer here agree with common sense? 147B. (Work together) Now consider the child’s motion after the initiation stage of the motion, 148 once he is already moving. 149 1. Intuitively, if the rope’s force remains larger than the child’s weight (like during the 150 initiation stage), does the child continue speeding up, or does he slow down, or rise with 151 constant speed? Briefly explain. 152 2. Does Newton’s second law agree with your answer? Explain. 153 3. Intuitively, if the rope force became smaller than the child’s weight, would the child 154 speed up, slow down, or rise at steady speed? Briefly explain. 155 4. Does Newton’s second law agree with your answer? Explain. 156 5. Let’s tie this all together. It makes sense that, if the rope force remains greater than the 157 gravitational force, the child keeps speeding up; and if the rope force becomes less than 158 the gravitational force, the child slows down. By this line of intuitive reasoning, what 159 happens to the child’s motion if the rope force equals the child’s weight, i.e., if the rope 160 force “compromises” between being greater than and being less than the child’s weight? 161 Explain. 162 6. Does Newton’s second law agree with your answer? 163 1645. Watch the video (about 1 min). The transcript is provided below. 165 Transcript: Green 3-5 166 167S4: Um, all right. 177S3: Like, if he goes from rest to motion, then 168S1: It probably doesn't make sense... writing 178 he needs to have acceleration. 169 gibberish. 179TA: Yeah. So it's like a two-step process, 170TA: So, tell me what you're thinking of this 5, 180 there's getting it started, and then the forces 171 this is where we tie everything together. 181 can balance, and it'll just keep going. 172S3: Um, we said that if both of the forces are 182 Good. That makes sense to me. 173 equal, the child would maintain the 183S4: So at first, the forces aren't equal, but then 174 velocity that he was going. 184 they do. 175TA: Right, and to give him that velocity to 185S2: Right. 176 maintain, though... 186TA: Yeah. 187S4: Ah.

9 Page 3 of 5 10Department of Physics Video discussion 11Teaching Assistant Seminar 4/26/2018 188TA: I mean, this sorta... the rough intuition that 198 when you keep going. Once you think, 189 “if the forces are equal, how can it even go 199 yeah that's true. 190 anywhere?” that was true. If the forces 200S4: Yeah, that's the part that's tough, because 191 were equal right from the beginning, it 201 you just think of the rope pulling the same 192 never would have gotten started. 202 amount each time 193S3: It would have to stay, yeah. 203TA: Right. 194TA: But when you're lifting something like a 204S4: When technically... 195 bag of groceries, you might not even notice 205TA: It's just like a quarter of a second, or 196 it until you start to... when you first start 206 something, it's easy to... 197 pulling, you're pulling a little harder then 207 2086. Consider the following questions about the episode you just watched. 209 A. The TA in this clip does a fair amount of prompting the students, as when he says, 210 “Right, and to give him that velocity to maintain, though…” in a tone that expects a 211 contrast with what the students just stated. He also summarizes the physics argument for 212 them. Here are two ways of interpreting the TA’s behavior: 213 1. This is an easy argument and the TA is pretty sure the students have it down. He 214 doesn’t want to belabor the point and risk boring or condescending to the 215 students; he wants to give them the benefit of the doubt, and offer them a quick 216 and helpful summary. 217 2. This is not an easy argument, but the TA has no way to know that, since he 218 doesn’t really listen to what the students are saying. Instead, he skims over 219 difficult and important physics and deprives the students of the chance to practice 220 their reasoning on him. 221 Which of these resonates with you? How do you think the possible benefit of prompting 222 and giving summaries compares with the risks? 223 B. When S4 says “So at first, the forces aren’t equal, but then they do,” do you think he 224 understood that already, or does it seem like he is having that realization just at that 225 moment? How does that bear on your interpretation of the TA’s behavior? 226 2277. The students continue with the tutorial exercises below. Do them collaboratively with your 228 partners before we watch the next video clip. 229 230 A. (Work together) Consider this intuition refinement diagram.

Raw intuition A force is needed for motion ? ?

A net force is needed to A net force is needed to Refined intuitions maintain an object’s motion initiate or change an object’s (velocity) motion (velocity)

2311. Which of those two refinements were you using (perhaps unconsciously!) in part of the 232 tutorial about when the rope first starts to pull Timmy upward?

12 Page 4 of 5 13Department of Physics Video discussion 14Teaching Assistant Seminar 4/26/2018 2332. Which of those two refinements were you using (perhaps unconsciously) when the rope 234 was raising Timmy at constant speed? 2353. Which of those two refinements agrees with Newton’s second law? 236 B. (Work together) Way back on the first page of this tutorial, we saw what Newton’s 237 second law says about the child and the rope: To keep the child moving upward at steady 238 speed, the rope force must equal (not beat) the child’s weight. Given that you’d already 239 figured out the answer to this question in part I (page 1), what was the point, if any, of 240 part II of this tutorial (pages 2 - 3)? 241 1. What do you think your professor would say? 242 2. What’s your own opinion? (Be honest: It’s OK if you disagree with your 243 professor, and we want to hear what you think.) 244 245 2468. Watch the video (about 1 min). The transcript is provided below. 247 Transcript: Green 3-6 248 249S2: What would Dr. Redish say? 259S4: Well, technically we were, like... 250S3: About what? 260S2: How do you know that your intuition is right 251S2: I don't know, I guess if you got the answer 261 without working it out? Right? 252 (S1 and S3: Oh. I was, why'd she say that?) 262S4: I don't know. (?) 253 on the first page, why do you have to do all 263S4: Motion's... to reconcile 254 of it? To learn to think through your 264S1: I love the words you use(?) 255 problems? Prove your intuition was correct? 265S4: our common sense with 256S4: Ours wasn't correct, though, right? 266S2: To make sure you're right. And gain skills 257S2: No. 267 in problem solving. 258S3: child moving upward at a steady pace 268 2699. Consider the following questions about the episode you just watched. 270 A. What question do you think they are answering – what their professor would say, or their 271 own opinion? 272 B. Why do the other students laugh at S4’s use of “reconcile”? 273 C. What do you think of S2’s answer? 274 D. Describe an interaction you might have with this group at this moment if you were the 275 TA. 276

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