Stress is an applied force that causes a material to be deformed (bent out of shape.)
Elastic materials are able to return to their original shape after being deformed. For example, when you pull outward on the ends of a rubber band it stretches. When you release the ends, the rubber band returns to its original shape and size. Thus, the rubber band is said to be elastic.
Even a rubber band has an elastic limit. If the rubber band is stretched (deformed) too much, it doesn’t return to its original shape.
Study the two diagrams: Before Stress and After Stress. Make note of the rubber band’s length in each diagram. Then choose the After Stress diagram, A, B, or C, that indicates the stress was:
- less than the elastic limit of the rubber band
- greater than the elastic limit of the rubber band
1. Think!
The original length of the rubber band before the stress was applied reached the ruler’s third mark.
During stress, the length of the rubber band was greater than the ruler’s fifth mark.
The elastic limit of a material is the maximum force on a material that results in the material returning to its original shape after the force is removed.
Any stress less than the elastic limit will result in the material returning to its original shape.
Answer: Figure B indicates the stress was less than the elastic limit of the rubber band.
2. Think!
The original length of the rubber band before the stress was applied reached the ruler’s third mark.
During stress, the length of the rubber band was greater than the ruler’s fifth mark.
The elastic limit of a material is the maximum force on a material that results in the material returning to its original shape after the force is removed.
Any stress GREATER than the elastic limit will result in the material NOT returning to its original shape.
For materials like rubber bands, balloons, and elastic, such as in clothing, when deformed they are larger than the original size.
Answer: Figure C indicates the stress was greater than the elastic limit of the rubber band.
Science Investigation
Use a gummy worm instead of a rubber band.
1. Using a ruler, measure the length of the candy worm. Record this as the worm’s length before stress.
2. Stretch the worm, recording the length during stress.
3. Release the worm and again measure its length. Record this as the worm’s length after stress.
Summary
If the gummi worm returned to its original length after the stress was removed, then the stress was LESS than the elastic limit.
If the gummi worm DID NOT returned to its original length after the stress was removed, then the stress was GREATER than the elastic limit.)
Conclusion: From the results of your measurements, was the stress greater or less than the elastic limit of the gummi worm?