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

[zandperl.livejournal.com]

If there were friction, it would actually depend upon how strong the friction were compared to the masses, so you'd have to answer D for all.

[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.

[inahandbasket]

I did. hah!

[zandperl.livejournal.com]

:-D Kudos to you!

[spoonless.livejournal.com]

I missed the "no friction" statement as well. But I answered "moves down" for the first two and "insufficient information" for the third, since no matter what the coefficient of friction is in the first two cases, it will still move down.

[zandperl.livejournal.com]

Well, so long as we assume μ<1 - although we learn that in intro physics, in reality some materials have μ>1, such as rubber, or under special circumstances, such as two smooth sheets of aluminum which are perfectly clean or two smooth sheets of glass with a film of water between them.

[spoonless.livejournal.com]

Oh, you're right! For some reason, I never knew you could have mu > 1... I'm glad I stumbled onto this poll :)

For boxes, it would seem somewhat unrealistic to have a mu that large. But I guess the whole question was not intended to be realistic.