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...

[zandperl.livejournal.com]

Hah! The correct answer is A for all.

I only gave the third situation to my class, they were to find the value of the acceleration, and two students said to me either during class or after "shouldn't the system not move since m2>m1 ?" I was surprised in both cases considering the particular students, so I wanted to see how widespread the misconception was, as well as what caused it.

[q10.livejournal.com]

i think they're probably overgeneralizing from the case where the pulley is right above the boxes, Box 2 is resting on a surface, and the Box 1 is dangling from a rope draped over the pulley. in this case (if i remember how these things work - which is a big if) Box 2 can't move down (because it's supported) and so long as Box 1 is lighter it won't pull enough to lift Box 2.

[zandperl.livejournal.com]

Possibly that (what you describe was a HW problem), possibly from the Atwood Machine situation (both blocks hanging), or maybe from having Box 2 on a plane that inclines downwards to the left. I first gave them that situation w/o friction, and then gave them this situation w/ little enough friction that the system would move.

[imluxionverdin.livejournal.com]

Maybe I'm wrong and looking at this the wrong way, but it seems to me that your students got it wrong because they were thinking of a real life situation.

You know that in practice if you did this experiment, and put mass m1 on the table and attached mass m2 to it (with or without a pulley!) and hung that over the table, if m1>>m2, nothing moves. This is due to the friction between the heavy m1 and the table.

Male m2>>m1, i.e. attach a large heavy object to a light one sitting on a table, and drape it over the edge, and you know it pulls down the light object very quickly.

The situation which you describe where you have no friction is not seen very often.

If you could do this experiment, i.e. put a very large object on a table and somehow make it near frictionless, by putting some lubrication on it (a very smooth object on smooth ice would do, but that's difficult ... look at Curling), and then attach a very small object and watch as it slowly overcomes the first objects inertia and pulls it to the side ... that would be an interesting experiment and an eye-opener for your students.

[zandperl.livejournal.com]

Actually, I'm likely going to do that on Monday in class, or otherwise Thursday in Lab. We have some carts with reasonably smooth axles on their wheels and I can do that in class, or in lab we have air tracks where an unwheeled cart rides atop a cushion of air (much like a hovercraft) so there's even less friction.