Many of us have become good at coming up with simple physics questions that students will struggle to answer correctly, definitely before instruction, probably during and even after instruction.
I’m curious about the following:
I want to hear about a question that you think students will likely answer correctly before instruction, but are likely to do worse after some instruction. I’m curious what the question is and why you think students will fare worse after (at least initial) exposure to instruction.
Teach. Brian. Teach. 
How I’d generate such a question: think of a question that has a counterintuitive right answer. These questions students will answer incorrectly before instruction and – perhaps – correctly after. Then think of a tweak to that question where the right answer is intuitively correct, but the question sounds so similar to the counterintuitive question that you think “oh this is one of those ‘gotcha’ questions…”.
Generating such a question isn’t coming to me really quickly. So I’ll think on it some more.
Not my idea,
But I remember reading that when young children as asked:
“What happens when you cut a magnet in half?”
before they talk about magnets in grade school, they tend to answer correctly, however after they are told enough to be told about north and south poles, they tend to incorrectly answer that you would get a north and a south pole.
Another, not mine
Emden asked “Why do we have winter heating?”:
“The layman will answer: ‘To make the room
warmer.’ The student of thermodynamics will perhaps express it thus: ‘To import the lacking internal thermal energy.’ If so, then the layman’s answer is right, the scientist’s is wrong.”
A rather tricky one that stumps a lot of physicists proper.
I had one of these last week. It was “what is a VGA connector.” I’ve got a room full of gamer-geeks — I assure you that they are not confused about how to assemble their amped-up PCs. About half were sure they were right when they pointed to some other connector. The other half were visibly thrown off balance when they realized that they used to think they knew this but weren’t sure any more. We had some interesting conversations.
Best I can do about why: Last semester we had a basic fabrication course. We focus on well-crafted work, best practices, and a little bit of “electronics is a foreign language that sounds like English.” That means learning that Ethernet isn’t the same as Internet; that the hole in the wall where you plug things in isn’t called a “plug;” the “plug” is the thing on the end of the cable. Typically my students have not had the need to distinguish these before, and it draws their attention unexpected facets of everyday things.
So we assemble some D-shell connectors, leaning that the standard that supposedly governs RS-232 comms is not very standard at all. Each student makes half a cable, and they have to connect their cables together to make a whole cable that allows them to communicate via terminal emulators. I suspect that this disproportionate emphasis is partly the cause.
This semester, we have a very brief intro to computer hardware and networking. I ask the students to identify as many external computer parts as they can from a checklist. We get to “VGA connector,” they look at the D-shells on the back of the PC, and that’s when they look a little dizzy. Or they just point to the serial connector confidently.
One student told me that he used to know that, but was thrown off by last semester’s experience of thinking he knew computers quite well and then realizing there were all kinds of external computer parts that he’d never looked at closely before. It’s an odd feeling that I can relate to — never having noticed something right under my nose, because I wasn’t looking for it. When it’s brought to my attention, it destabilizes the things I already knew, since now I feel like the old knowledge belonged to some other object (even though the object is the same — it’s my perception that has changed). It feels like “getting new eyeballs.”