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

[gemini6ice.livejournal.com]

Ooh, good point. Well, the friction would depend (linearly?) on the weight of box 2, right, so if friction enters the picture, there would definitely be SOME ratio of heavy/light between the boxes that would ensure movement. So if we say "2 is heavy and 1 is light," we can assume that they are SUFFICIENTLY heavy and light to overcome the friction. So I claim we would get A for the first. But still D for the second and third.

[zandperl.livejournal.com]

Well, the friction would depend (linearly?) on the weight of box 2, right,

Right.

So I claim we would get A for the first.

Not necessarily, if there's a LOT of friction and m1 (hanging) is only a smidge heavier than m2 (table).

FWIW, the specific situation I gave my students had Box 2 (table) heavier than Box 1 (hanging), *with* friction, and yes they moved (at around 1/3 of g, even). I don't have the numbers on me right now.

[gemini6ice.livejournal.com]

Not necessarily, if there's a LOT of friction and m1 (hanging) is only a smidge heavier than m2 (table).


Right. But I feel if we are given, "this is heavy and this is light," we can assume they are sufficiently heavy and light. It's just a difference of opinion over what's a reasonable assumption for the way the problem is stated, just like rosefox feels that gravity is not reasonable.