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

[dlakelan.livejournal.com]

what do the students think? please don't say they think the hanging one moves up...

[inahandbasket]

oops, failed to see your "assume no friction."
Change all my answers to A.

[calzephyr77.livejournal.com]

I'm scared to answer because I didn't take high school physics, but I'll give it a shot :-)

[littlez-star.livejournal.com]

I liked this questionnaire, and I'm very curious about the right answers. Will you post them up?

[chipuni.livejournal.com]

For a (hypothetical) fourth question...

May I fill box 1 with hydrogen gas? Pleeeeeease?

[sethrates.livejournal.com]

I'll need to reserve judgment until I've run 50 billion monte carlo simulations on computers distributed throughout the world, and had all of them fail because of coding errors.

[rosefox]

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

[sirroxton.livejournal.com]

Teaching is hard.

Most textbooks are terrible. Authors accumulate a lot of knowledge over a lifetime, and then think, "Oh, if only it had been broken down for me *this* way, it would have been easy to understand." But the way people learn doesn't map well to these after-the-fact hierarchical breakdowns.

So it's always heartening to see examples like this, with teachers really digging at those fundamental, hard-to-articulate questions of how people think and how to convey new ways of making meaning to those who haven't internalized those ways. (It's easy to teach students who already know a lot of the critical ways of learning things; that's the teaching equivalent of grunt work.) Keep digging, and cheers. :)

[kelsin.livejournal.com]

Having taken physics I think the problem was stated fine. I understood it perfectly. It did take me a second to think of the answers but I got them all right after about 10 seconds of thought on each. I just made myself think of the forces, and in all cases you have a downward force on box 1 and nothing on box two (since gravity on it doesn't do anything since it's perpendicular to the box and there is no friction.

Obviously like you're saying above, even if there was friction the answer would still likely be A for all situations, but when you simplify it (as in the problem) you don't even have to worry about the weights.

So free body diagram ftw?

[catalyst37.livejournal.com]

Well it is an easy question. You would think that on a frictionless surface you would have any force cause box two to move. You should let your students think of it in realistic terms instead of abstract or mathematical if they can not fully grasp them. You should also try and get them to break down the whole question into a couple smaller ones then try to connect them. I am a firm believer that most students who do poorly in any subject is merely laziness in thought. If they can not figure out the question immediately or at least have the logical path of figuring it out they do not try to THINK it through, instead they cut the loss of marks and move on to the next. I know this because I once thought like this, so hopefully this gives you some insight into how your weaker students think. They need to know that this is in most cases the reason for not completing a question and once they know or realize that subconscious process they can correct it.

I hope this helps
Alexzander

[sirroxton.livejournal.com]

I think part of the problem might be that this setup just doesn't make sense in a physical context to someone who doesn't understand friction.

We look at this diagram, and we have an intuition about it. But our intuition has everything to do with friction, and we don't understand anything about friction in mathematical terms -- it's a frustrating bit of voodoo. Personally, I remember having a serious eureka moment when I learned about the distinction between static and kinetic friction.

Usually when teachers present something intuitive, it's supposed to be a springboard to understanding a general concept. In this case, the intuition is just confusing.

embarrassing

[michimusic.livejournal.com]

I wasn't thinking about friction, and I answered "C" on the last one. Even more embarrassing - I have a minor in physics. Intuition sucks. Guess it's good I have a day job.