OK, a shot at Challenge I, with Poof and Foop, Steam Locomotive, and Expansion of Nothing. Felt like all three are in the sweet spot. I personally dislike Expansion of Nothing.
Poof and Foop:
The problem statement is a bit leading: there’s some kind of inversion symmetry relationship between the two cases, so it should go the opposite direction, right?
Initially, definitely. The puncture means that there’s less pressure on the right side—instead of colliding with the can, some particles go inside.
But those particles end up colliding with the interior left side anyway. So it seems like it should even out, and at the end the can won’t be moving.
So my guess is (c). Can I make myself more confident?
Why doesn’t an inversion argument go through? Well, the compressed air can is drawn in vacuum, but the vacuum can doesn’t empty the environment. So it’s not simply time reversal. If the compressed air can were in air, then we might have some kind of symmetry between air particle and absence of air particle, but then the compressed air can would slow down due to drag and stop in the limit. So that still points to (c). That also works as a thermodynamic argument—the first can isn’t equilibrating with anything, so none of the work goes to heat. 95% confidence feels good.
*checks* OK, looks like I was thinking about it right, and my explanation for why the naive inversion is wrong is equivalent to sketch II.
Reflection: The main interesting thing here is the fake symmetry argument. My favorite problems have tempting solutions that don’t work for subtle reasons. I think it’s important not to count problems solved until you can pinpoint why those solutions fail.
What did I use here? If you’re dealing with pressure, you can probably get an answer with forces or with thermodynamics. A net force can be thought of as a single force or as lack of a balancing force. That’s the symmetry idea.
I’m not very good at babbling. I’m basically looking over what I wrote and renarrating it. Words to words.
Steam Locomotive:
We might want to think about torque and the height of the axle. Or maybe it’s about wheel radius. One cycle takes you further with bigger wheels.
I think these both point to (b). I’m a little confused because thinking about the wheel heights of sports cars and trucks would push me towards (a). But cars have gears. Directly driving small wheels is basically low gear. Not sure how I’d know if the answer were (c) or (d). Seems like you’d need background knowledge not in the question. I should think about actual forces to get to 95% confidence.
Let’s say the engine puts out the same force in both cases. Then, in II, each wheel sees half as much force from the engine, but the ground exerts force on twice as many wheels, so that part’s a wash. But because the wheels are smaller, the ground needs to exert more force per unit engine force to keep the wheel from slipping (same torque).
So for the same engine, II seems to give more accelerating force, while I gives higher top speed. I’d put 95% on (b).
*checks* OK, seems like I had the right thought. Could I have been as confident from the distance-per-cycle argument alone? Rather than look at forces, the author’s answer argues that we know the locomotive that goes a shorter distance in the same number of engine cycles must be putting more energy into each mile it travels. I considered that, but I wasn’t sure it was a valid step. Why couldn’t you just be getting less work from the engine? Well, it’s the same piston with the same motion. My force calculation already needs that assumption, it just makes the final connection with the acceleration.
Reflection: I feel like I don’t know much about automotives. (Is a locomotive an automotive, by the way? I think so, it’s just locomotives involve a track.) I can describe transmission and gears and engines and so on if I think about it, but I don’t have much intuition. Like, I can’t explain why it’s one way and not another, or how different cars solve different problems.
I just feel like I should have been able to answer the question immediately. If I could drive stick, would that help? Probably not. I already ride a bike and didn’t immediately see the analogy.
What did I use? Qualitative personal experience. I picked a misleading experience but reasonably didn’t weight it above thinking through the problem. Identifying relevant considerations. Didn’t stop at the first idea.
Expansion of Nothing:
Oh, this one’s nasty. It has to expand, right? If you took an iron disk and drew a circle where the hole is, the circle would expand. If you cut that disk out and heat up the cutout, the disk expands the same amount. So everything outside the circle can’t be exerting any net force at the boundary, and the hole has to stay the same size as the disk.
I don’t see any problems with this argument, but can I explain why other arguments don’t work? Why can’t thermal expansion generate stress instead of allowing uniform expansion? I guess in a sense I just gave the reason, but why does the gap shrink if you cut a gap in a rod instead? Well, when you have only one piece, it’s like applying a magnification transformation, which requires an origin. But the origin is arbitrary—you can just recenter. With two separate pieces, the two origins of magnification are no longer arbitrary.
*checks* Yeah, the author’s answer doesn’t go there, unfortunately.
Reflection: This problem feels really annoying to me. Maybe I saw it a long time ago and got it wrong? Or maybe it’s that you never have anything that’s free to expand uniformly. It’s braced against something, or it’s sitting on something with a different coefficient of thermal expansion, and you do get stress and it does matter how the thing is patterned.
This feels like a problem where you’re supposed to think about limiting cases. Like, if you have an atomic ring, obviously it expands. I don’t know if you can justify jumping to the right answer from that, though. If the disk is thick and the cutout doesn’t go all the way through, it expands. Ehh. You still need an argument that it expands the same.
I’d also add: a TODO item on my list is to make my own followup question for Expansion of Nothing that presents rings of different materials (i.e. something like a ring of water, a ring of jelly, a ring of concrete, something like that), and asks “in any of these cases, do you get a different answer than the Iron Ring?
I don’t actually know currently whether there exists a material that gets you a different answer, so this is a bit of a research question. Crafting the final question such that it somehow gives you enough information feels like part of the exercise. [by which I meant the meta-exercise of “creating an exercise”, which I think is also a useful thing to learn]
I enjoyed it, although I’m already the sort of person who thinks Thinking Physics is fun—both the problem solving and the nitpicking about what constitutes a correct explanation. It seems worth doing at least a handful of problems this way, and more broadly deliberately practicing problem solving and metacognition about problem solving. Thinking Physics could be a good complement to Problem Solving Through Problems or How To Solve It, since in my (limited) experience you get quickly diminishing returns to anything but competition math with collections like that.
OK, a shot at Challenge I, with Poof and Foop, Steam Locomotive, and Expansion of Nothing. Felt like all three are in the sweet spot. I personally dislike Expansion of Nothing.
Poof and Foop:
The problem statement is a bit leading: there’s some kind of inversion symmetry relationship between the two cases, so it should go the opposite direction, right?
Initially, definitely. The puncture means that there’s less pressure on the right side—instead of colliding with the can, some particles go inside.
But those particles end up colliding with the interior left side anyway. So it seems like it should even out, and at the end the can won’t be moving.
So my guess is (c). Can I make myself more confident?
Why doesn’t an inversion argument go through? Well, the compressed air can is drawn in vacuum, but the vacuum can doesn’t empty the environment.
So it’s not simply time reversal. If the compressed air can were in air, then we might have some kind of symmetry between air particle and absence of air particle,
but then the compressed air can would slow down due to drag and stop in the limit. So that still points to (c). That also works as a thermodynamic argument—the first can isn’t equilibrating with anything, so none of the work goes to heat. 95% confidence feels good.
*checks* OK, looks like I was thinking about it right, and my explanation for why the naive inversion is wrong is equivalent to sketch II.
Reflection: The main interesting thing here is the fake symmetry argument. My favorite problems have tempting solutions that don’t work for subtle reasons. I think it’s important not to count problems solved until you can pinpoint why those solutions fail.
What did I use here? If you’re dealing with pressure, you can probably get an answer with forces or with thermodynamics. A net force can be thought of as a single force or as lack of a balancing force. That’s the symmetry idea.
I’m not very good at babbling. I’m basically looking over what I wrote and renarrating it. Words to words.
Steam Locomotive:
We might want to think about torque and the height of the axle.
Or maybe it’s about wheel radius. One cycle takes you further with bigger wheels.
I think these both point to (b).
I’m a little confused because thinking about the wheel heights of sports cars and trucks would push me towards (a). But cars have gears. Directly driving small wheels is basically low gear.
Not sure how I’d know if the answer were (c) or (d). Seems like you’d need background knowledge not in the question.
I should think about actual forces to get to 95% confidence.
Let’s say the engine puts out the same force in both cases. Then, in II, each wheel sees half as much force from the engine,
but the ground exerts force on twice as many wheels, so that part’s a wash. But because the wheels are smaller, the ground
needs to exert more force per unit engine force to keep the wheel from slipping (same torque).
So for the same engine, II seems to give more accelerating force, while I gives higher top speed. I’d put 95% on (b).
*checks* OK, seems like I had the right thought. Could I have been as confident from the distance-per-cycle argument alone? Rather than look at forces,
the author’s answer argues that we know the locomotive that goes a shorter distance in the same number of engine cycles must
be putting more energy into each mile it travels. I considered that, but I wasn’t sure it was a valid step.
Why couldn’t you just be getting less work from the engine? Well, it’s the same piston with the same motion.
My force calculation already needs that assumption, it just makes the final connection with the acceleration.
Reflection: I feel like I don’t know much about automotives. (Is a locomotive an automotive, by the way? I think so, it’s just locomotives involve a track.) I can describe transmission and gears and engines and so on if I think about it, but I don’t have much intuition. Like, I can’t explain why it’s one way and not another, or how different cars solve different problems.
I just feel like I should have been able to answer the question immediately. If I could drive stick, would that help? Probably not. I already ride a bike and didn’t immediately see the analogy.
What did I use? Qualitative personal experience. I picked a misleading experience but reasonably didn’t weight it above thinking through the problem. Identifying relevant considerations. Didn’t stop at the first idea.
Expansion of Nothing:
Oh, this one’s nasty. It has to expand, right?
If you took an iron disk and drew a circle where the hole is, the circle would expand.
If you cut that disk out and heat up the cutout, the disk expands the same amount.
So everything outside the circle can’t be exerting any net force at the boundary, and the hole has to stay the same size as the disk.
I don’t see any problems with this argument, but can I explain why other arguments don’t work? Why can’t thermal expansion generate stress instead of allowing uniform expansion? I guess in a sense I just gave the reason, but why does the gap shrink if you cut a gap in a rod instead? Well, when you have only one piece, it’s like applying a magnification transformation, which requires an origin. But the origin is arbitrary—you can just recenter. With two separate pieces, the two origins of magnification are no longer arbitrary.
*checks* Yeah, the author’s answer doesn’t go there, unfortunately.
Reflection: This problem feels really annoying to me. Maybe I saw it a long time ago and got it wrong? Or maybe it’s that you never have anything that’s free to expand uniformly. It’s braced against something, or it’s sitting on something with a different coefficient of thermal expansion, and you do get stress and it does matter how the thing is patterned.
This feels like a problem where you’re supposed to think about limiting cases. Like, if you have an atomic ring, obviously it expands. I don’t know if you can justify jumping to the right answer from that, though. If the disk is thick and the cutout doesn’t go all the way through, it expands. Ehh. You still need an argument that it expands the same.
I actually created a doc where people can add their own confusions and answers for Expansion of Nothing: https://docs.google.com/document/d/1cleM-QuO9R9_jRqDZMMKzobpWcf-k9KHBe91fUMWhuQ/edit
I’ll edit it into the OP.
I’d also add: a TODO item on my list is to make my own followup question for Expansion of Nothing that presents rings of different materials (i.e. something like a ring of water, a ring of jelly, a ring of concrete, something like that), and asks “in any of these cases, do you get a different answer than the Iron Ring?
I don’t actually know currently whether there exists a material that gets you a different answer, so this is a bit of a research question. Crafting the final question such that it somehow gives you enough information feels like part of the exercise. [by which I meant the meta-exercise of “creating an exercise”, which I think is also a useful thing to learn]
I’m also generally curious how you found the exercise, whether it seemed worthwhile to you.
I enjoyed it, although I’m already the sort of person who thinks Thinking Physics is fun—both the problem solving and the nitpicking about what constitutes a correct explanation. It seems worth doing at least a handful of problems this way, and more broadly deliberately practicing problem solving and metacognition about problem solving. Thinking Physics could be a good complement to Problem Solving Through Problems or How To Solve It, since in my (limited) experience you get quickly diminishing returns to anything but competition math with collections like that.