Help me understand a misconception....

[asterroc]

An interesting situation came up in my Physics class today, where two of my students surprised me with a question they asked. To try and understand their thinking so I can teach the content better, I'd like to ask that everyone take a look at the below situation and tell me what you think will happen. I don't care if you know any physics or if you're a professional ear-wax taster, I want to know what you think and why.

In the picture below, Box 1 (m₁) is hanging from a string that passes over a pulley. There's no friction in the pulley, and the pulley has no mass, so it can spin freely. The string is then connected to Box 2 (m₂) sitting on a table. For simplicity, let's assume there's no friction on the table - there's some lubrication between the box and the table.



[Poll #1162218]

X-posted a couple places.

Comments

[rosefox]

I really, really hate "ideal world" puzzles, and this sort of thing is exactly why.

[zandperl.livejournal.com]

Heh. I think the actual situation I have my students *did* have friction, and Box 2 was more massive than Box 1, and it *still* moved. It depends on how much friction there is compared to how big m1 is.

[rosefox]

It's not even the friction, it's that as you describe it, box 1 appears first, apparently floating in midair, and then a string is attached to it and put over the pulley, and then box 2 appears and the string is attached to box 2. Since box 1 was already floating in midair, I can't think of any reason it should stop.

EDIT: Specifically, you say that box 1 is hanging from the string. That implies that the string is holding it up. I can't imagine how the string would do that, with the frictionless pulley there, but that's what's stated, so I believe you.

[zandperl.livejournal.com]

"Hanging" doesn't necessarily mean "hanging AND stationary". "Hanging" means "there is a force in this rope," it says nothing about the object's velocity or acceleration.

I don't think this specific source of your confusion is what is affecting my students in this case, as that meaning of "hanging" had been established in the course by then. But other parts of what you said do remind me to choose my words carefully. And now that I think about it, I think the two students who asked me the question are non-native speakers of English, so that might be affecting their understanding too.

[q10.livejournal.com]

i have to say that in most ordinary English language contexts the word ‘hanging’ seems to mean more than that.

[zandperl.livejournal.com]

*nods* I definitely get that, and I definitely see where that (and other wording choices I gave here) confuses Rose. However, as mentioned, I do not think this is what threw off my students because that usage has been established by now.

Based on comments here and to the post in physics my guess is that there are two things that caused the confusion. (1) In a situation like this in reality, chances are that the friction will be great enough that the system will not move, therefore students will generalize this to think that the system will NEVER move. I could overcome this next semester by instead asking them to calculate a range of values for the friction for the system to move. I will overcome this THIS semester with a physical demonstration in class next week.

(2) "Infinite Inertia" - I'm not sure that students get that somewhere way out in space, any non-zero force will cause an object to begin accelerating. Hopefully one of the demos this week will help that a bit, but I'm not sure it will, so I'm open to suggestions there.