# God I Need help or (Rubber band stretching)

## do you know?

Poll closed Dec 15, 2003.

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1. Dec 10, 2003

### jungboho

rubber BANDS

hi guys
i was just wondering what the relationship between the weight and stretch length of rubber is. And also why does an 8cm rubber band increase it's stretch length by 5cm at 600gms weight while the trend before and after is every 10gm added to the weight adds 0.2 to to the lenght.

2. Dec 10, 2003

### Integral

Staff Emeritus
could you post your data? I am not sure I understand what you are seeing.

3. Dec 10, 2003

### jungboho

ok
it may take me a while to obtain it

4. Dec 10, 2003

### StephenPrivitera

I'm not sure what you're getting at, but I noticed once that if you take one of those strings you sometimes attach to light bulbs and stretch it 3 or 4 inches, it will hit the ceiling when you release it. There must be a great deal of potential energy associated with the string. I haven't played with rubber bands recently, but my guess is they act similarly. They obviously don't obey hooke's law.
I think that's what you're getting at anyway.
You should be able to determine the relationship to some degree yourself through experiment.

5. Feb 25, 2004

### jungboho

God I need help

I have been having trouble with this for ages

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6. Feb 25, 2004

### jungboho

i'm back with my data

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7. Feb 25, 2004

### Staff: Mentor

One data point off in the middle of nowhere is usually a sign of error.

8. Feb 25, 2004

### NateTG

Can you provide more context?

9. Feb 25, 2004

### Loren Booda

Elasticity can be assumed to first approximation to obey F=kx, the "spring" equation. Replacing the force F with mg, we have mg=kx, where m is the hanging mass, g the local acceleration of gravity, k is the spring constant, and x is the displacement downward. Since g and k are considered constant, m varies directly as x.

10. Feb 25, 2004

### Integral

Staff Emeritus
What do you need to do with it?

Looks like you have an error (perhaps droped decmial point) should that "outlayer" be .6 rather then 6?

Looks like you have the data in Excel, you can do a linear regression to get the best fit line. I would use the data from about 200 up, as below that there is very little change.

As I Recall you will need to learn to create an ARRAY FORMULA to use the Linest function.
select 2 adject cells and enter
=LINEST(A6:A19,B6:B19)
replace the specified ranges with your data location.

prese ctrl-shift-enter.

This will give you the slope and intercept of the data.

11. Feb 25, 2004

### Loren Booda

After the linear elastic region, there is a critical point in strain whereafter nonlinearity leads to failure of the specimen.

12. Feb 26, 2004

### Spectre5

yea, are you sure you did not mean to have that value .6??

13. Feb 26, 2004

### jungboho

yep incredibly sure

I have repeated the experiment a few times the jump always happens though in varying amounts.

14. Feb 26, 2004

### jungboho

email me at jungboho@hotmail .com for the full method and excel document

15. Feb 26, 2004

### jungboho

16. Feb 26, 2004

### Staff: Mentor

That's true, but the critical point is at a the point of a shape change of the curve, not a transiant jump in one value. The curve remains continuous.

17. Feb 26, 2004

### jungboho

strange isn't it
This was physics coursework but ow it's transcended into something more

18. Feb 26, 2004

### Integral

Staff Emeritus
It seems that you are plotting the CHANGE in length. Since the basic relationship is f=-kx it would be more meaningful to plot the total length, that would change your spike to a step, much easier to understand. Also it would give physical meaning to the slope of your data.

19. Feb 26, 2004

### Loren Booda

jungboho
Ow indeed! Welcome to the universe of Physics.

20. Feb 26, 2004

### pallidin

Hmmm. I was under the impression that a uniform rubber band requires increasing force to stretch it further than its previous point. So, to suggest that a certain amount of force always increases length by a constant amount after a given stretch point makes no sense to me in my experience with rubber bands.
Indeed, does not a rubber band increase its resistance to extension the more it is pulled?
Anomalies to this might be encountered if the rubber band was not dimensionally uniform(what rubber band truly is), or if the rubber band in processing has variant elasticsity due to poor manufacturing(common; look closely at a band and one sometimes finds regions of more dense rubber), or random chemical contaminants which change local elastic properties.