B Rubber band force-extension graph

[Aimhigh]

TL;DR Summary

The change of length during unloading is greater than during loading for a given change in tension

My AQA Year 12 physics textbook shows the following graph shown below

Then it says: "For a rubber band, the change of length during unloading is greater than during loading for a given change in tension."
I don't understand this.
In the graph below, it looks to me that for a certain change in force, the extension is bigger for the loading curve rather than the unloading curve.
Please help me understand this.

 

[berkeman]

Are you familiar with the concept of hysteresis?

 

[Aimhigh]

No I am not, sorry. But I could look it up

 

[berkeman]

This will get you started: https://en.wikipedia.org/wiki/Hysteresis

It basically happens when a material has "memory" of recent things that have happened to it (like being stretched, or a magnetic material that has been magnetically polarized one way and then is subject to reverse polarization).

 

[Aimhigh]

Thank you
I think the book is not worded very clearly.

According to the wikipedia page you sent me, what I have come to understand is:
"The effect can be demonstrated using a rubber band with weights attached to it. If the top of a rubber band is hung on a hook and small weights are attached to the bottom of the band one at a time, it will stretch and get longer. As more weights are loaded onto it, the band will continue to stretch because the force the weights are exerting on the band is increasing. When each weight is taken off, or unloaded, the band will contract as the force is reduced. As the weights are taken off, each weight that produced a specific length as it was loaded onto the band now contracts less, resulting in a slightly longer length as it is unloaded."

I think what the book should say is: As the band is unloaded, the extended length for a particular weight is larger than the extended length for that same weight when it was being loaded.

 

[berkeman]

I think the book is not worded very clearly.

I agree.

I think what the book should say is: As the band is unloaded, the extended length for a particular weight is larger than the extended length for that same weight when it was being loaded.

I believe that is correct, although I'm much more familiar with hysteresis in magnetic materials (like for transformers) than I am with rubber bands. It's basically a "memory" effect.

 

[sophiecentaur]

Hanging masses on a spring or rubber band is very convenient and suitble for school lessons. The 'independent variable' in that case is the load and the dependent variable is the extension (length). However, there is a snag with high loads.The test piece gives up the ghost when the load is just enough and all you can see is the masses falling on the floor. A far better method is to 'stretch the test piece and measure the force needed. That reveals a lot more about the strength under high loads and they are applied and then reduced. This link makes good reading and it a higher lever than regular school Physics but it is worth reading. This graph from the article shows how a sample behaves beyond the point where 'the spring stretches and breaks all in one instant'.