Does Teaching by Numbers Limit Understanding of Elasticity in Rubber Bands?

[Josh0768]

If you take a rubber band and fix it in a stretched position for an extended period of time, would it eventually lose its elasticity? If yes, then how can you calculate how long it would take until its elasticity decreases by a certain amount, say, fifty percent? If no, why not? How does the rubber band constantly maintain elasticity?

 

[anorlunda]

Rubber bands become dry and brittle with time.

How old are you? My guess is that you don't find 50 year old rubber bands in the drawers of your office.

 

[Keith_McClary]

My guess is that you don't find 50 year old rubber bands in the drawers of your office.

I saved the rubber bands that came wrapped around the daily newspaper. How old does that make them/me? I just now tried stretching a few, and only one broke and zapped my fingers.

The science of Rubber elasticity.

 

[hmmm27]

There's an easy way to test :

Hang the rubber band from a hook next to a wall.
Hang a weight on the rubber band.
Mark the wall.
Wait.

 

[Keith_McClary]

Hang the rubber band from a hook next to a wall. Hang a weight on the rubber band. Mark the wall. Wait.

The weight will move up or down with the temperature.

 

[davenn]

There's an easy way to test :

Hang the rubber band from a hook next to a wall.
Hang a weight on the rubber band.
Mark the wall.
Wait.

The weight will move up or down with the temperature.

a tiny fraction, but nowhere near as much as it's stretching due to the loss of elasticity

It's a good and easy practical test

 

[sophiecentaur]

We learn Hooke's Law at school but it's only later that we seriously examine what happens to rubber bands (and plastic bags). That S shaped curve of force against extension of a rubber band is easy to obtain with a home experiment. It's a quantitative confirmation of the way a rubber band goes very stiff just before it breaks. (and new balloons are very difficult to get started when you blow the up.

Even steel springs start to creep, though. The spring in an airgun will need replacing after a lot of use and people feel it's necessary to maintain the 12ft lbs that's the maximum allowed.

 

[Keith_McClary]

Hang the rubber band from a hook next to a wall.
Hang a weight on the rubber band.
Mark the wall.
Wait.

In the first couple of hours (from original stretched length ~87 cm of 5 bands looped to make a chain) it stretched ~6 cm and after a few days another ~10 cm. It still bounced up and down nicely. What do we mean by "elasticity", exactly?

(The next day it had broken.)

 

[anorlunda]

There are no language police. Words like elasticity can mean whatever the author wants.

 

[sophiecentaur]

There are no language police.

That’s not always what we observe on PF.

 

[jrmichler]

What do we mean by "elasticity", exactly?

That is not a simple question. If you really want to know more, you can search elastomer mechanical properties modulus creep. This hit is a good overall look at elasticity of rubber bands and other elastomers: https://apps.dtic.mil/dtic/tr/fulltext/u2/273880.pdf. It's a little old, but the information is still good. Figure 2.2.1 shows a simple model for the response of elastomers that gives an idea of why this is not a simple question.

If you really want to learn more, search elastomer material models. If you start to feel like you are trying to drink from a fire hose, you are.

 

[Keith_McClary]

If you start to feel like you are trying to drink from a fire hose, you are.

 

[sophiecentaur]

That is not a simple question.

It's an example where teaching by numbers (which is all there's time for in school) can let people down when they want to move on. Those textbook graphs are very often just not enough.