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

[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?

[zandperl.livejournal.com]

The students in question were capable to doing FBDs, but couldn't get past their intuition that the objects shouldn't move. When your intuition says something so strongly, you make bad assumptions and can't get any further - they probably wrote things like σF=0 when attempting to do the math, for example.