The factors, which affect the extension of elastic, are;
- The amount of force applied
- The thickness of the cord
- The length of the cord
- The width of the cord
- The temperature of the cord
The elastic cord is made of rubber, rubber is a special type of molecule called polymer. A polymer is a long chained molecule. The arrangements of this molecule are not ordered and infact they double up back on themselves and become tangled.
Therefore, when rubber is stretched, the tangles are straightened out, a small force is needed to do this, as the tangles are cross-linked.
When chains are untangled and straightened out, extension can only take place if the molecules in the polymer chain are separated, however this is difficult because molecules are held strongly and with lots of attractive forces between them. Therefore the more force applied to the elastic cord, the more it will stretch initially however then it will become harder to extend even if more and more force is added.
The thicker the cord, then the greater the number of polymer chains to untangle and this is why the cord is difficult to stretch as it is made thicker.
The same method applies to increasing the width of the cord as when you increase the number of polymer chains etc.
The temperature also affects the kinetic energy of molecules and how temperature of kinetic energy is less. Therefore polymer chains are tightly held and are difficult to pull apart. Temperature increases kinetic energy molecules as they begin to move more, therefore polymer chains can be untangled and pulled apart more easily.
The factor I am going to investigate is the affect of force on the extension of a rubber cord.
Prediction:
I predict that as the amount of force on the cord is increased, the cord will begin to stretch. I do not think that the cord will obey Hookes Law- that basically means the extension will not be proportional to the force applied. This is because the cord is made of rubber and the molecular structure of the rubber is very different from other materials such as metal wires and springs. I am going to investigate the affect of force on the extension of the rubber cord in the following way.
Method:
- Set up experiment as shown in the diagram.
- Measure the length of the elastic
- Add the force of one Newton
- Measure the length of the elastic now
- Continue steps 3-4 adding one extra Newton each time
- Repeat the experiment 3 times for a fair test.
- 1 Newton is equal to 100grams
- Measure using a millimetre ruler
Analysis and Conclusion:
My results show me that as you increase the force, the extension also increases.
The graph is also curved and slightly “S” shaped which shows that it doesn’t obey Hookes Law. Hooke’s law states that if you double the force, the extension will also double and therefore be proportional to the force.
1N 0.16 This shows that Hookes Law is not present in these
2N 0.9 results
4N 3.06
8N 9.23
- What is the structure of elastic?
- At first 1N- 2N force is not big enough to untangle the polymer and therefore it stretches.
- Between 3N and 8N is a big enough extension because the polymer chains are being untangled.
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9N or more and the chain is untangled. Extension is small because F is trying to pull molecules apart and break bonds, which are very strong etc.
My results did not obey Hookes Law and therefore my prediction was correct. I knew that the elastic would not obey Hookes Law because of the molecular structure- with other materials such as metal, Hookes Law would’ve been enforced.
Evaluation:
My experiment proved my prediction as I stated that the Elastic would not obey Hookes Law.
If I were to conduct this experiment again, I would have to say that I would use different elastic bands for each experiment. This is because I feel that a fair test was not fully achieved due to the fact that after each experiment, the elastic may have been slacking slightly. I would also insist on having a pointer attached to the elastic, which would make it much easier reading the extension and therefore giving more accurate results.
I would also repeat the experiment more than 3 times and work out an average of the three to give a more reliable even though my results were reliable enough to prove my prediction.
If I were to repeat this experiment, I would go beyond 10N and would be very interested to see what the results of that experiment would be and to see whether any major changes have occurred in my prediction and results.
We used corded elastic in this experiment, again, I could make a change if I were to re-conduct this experiment and use a rubber band instead of the corded elastic.
I feel that because of all of these factors noted above, my experiment was not a fair test, although I feel that it was reliable enough to prove my prediction correct.
By Sanjay Virdi- 11BG/CM
