Rubber Bands and Heating: Does Weight Go Up or Down?

[daniel_i_l]

Suppose you have a weight hanging from a rubber band and you heat up the rubber band, will the weight go up or down? from grade-school the answer "down" was pounded into my head - obviously - when something is heated it expands! but yesterday i was reading the Feynman lectures and in the chapter about statistical mechanics he said that the rubberband actually gets shorter when heated. he explained that heating the RB gives it more tension and so it pulls harder on the weight and the weight goes up. but isn't it true that as something is heated it expands?
Thanks.

 

[Hurkyl]

isn't it true that as something is heated it expands?

Not universally. A more common example is ice: heating ice will cause it to shrink. And the water you get will continue to shrink as heated, until about 4° celsius.

Heating something causes its molecules to become more energetic. More energetic particles tend to remain further apart than less energetic particles (thus expanding the object), but sometimes other considerations predominate.

For basic ice, it's because water molecules tend to organize into a sparse crystalline structure. When heated, the crystal dissolves and the molecules arrange themselves in a more random (and thus denser) fashion.

I don't know what's going on for a stretched rubber band. My first guess is that it wants to expand laterally, which makes the rubber band feel more stretched, and thus it tugs harder. I have no reason to believe that's right, though.

 

[Danger]

I guess that it would depend upon the composition of the band. Generally, I would expect it to stretch farther since rubber softens when heated. (Don't ask me how I know this; let's just put it off as a bad experience...)

 

[Bystander]

https://www.physicsforums.com/showthread.php?t=121620

 

[Gokul43201]

I guess that it would depend upon the composition of the band. Generally, I would expect it to stretch farther since rubber softens when heated.

Not for small increases in temperature (Le Chatelier). For most common types of rubber (polyisoprene based) the typical deflection temperatures or maximum working temperatures are from about 50C to about 250C, so it would take some serious heating to get the rubber to soften to its yield point.

PS: Some related discussion here: https://www.physicsforums.com/showthread.php?p=419641#post419641

 

[Danger]

Hmmm... interesting.