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Well, the value of k changes depending on the length of your cord, even with the same material. You'd need a different formula to figure out a constant that also factors in a changing length (hint hint, one exists).
If you are using rubber bands:
1. Make sure that your cord can meet the test for elasticity. The bottom 1 meter should stretch to 1.25m with a 500g mass attached to the bottom.
2. The main problem I foresee with rubber bands is consistency in the elasticity of the rubber bands. Due to the nature of rubber bands, there will be some variation in their elasticity. This will lead to a cord that's more elastic in some parts and less elastic in other parts. This could lead to errors in you calculations.

One more thing: testing is huge in this event. Knowing beforehand how well your equation matches up with reality helps you a ton in the competition.

reedm121 wrote:so I should find the spring constant (k) before hand with trials and such and then during the competition plug in the F value (which I could calculate from the mass) into the equation F=kx to find x, which I could use to determine the height?
Also is it okay that I made my "bungee" out of rubber bands?

My team's bungee is made of rubber bands and it works fine. Furthermore, i wouldn't suggest using F=kx for this since it wont be as useful. However, as a word of advice, think in terms of energy. I derived equations for this event and achieved accuracy with max error of 2 cm. And usually less than 1 cm. You really only need one good trial to calibrate your equation and then you should be set.

Also, people should be careful about how many times they test their bungee. The more you test, the less elastic the cord becomes and it skews your earlier data /equation. Ideally you would have few tests and use your stastical knowledge to masterfully interpolate, or you would have one perfect test and use an equation.

So on average, what's your guys' average distance from the ground?
My team is consistently under 5cm, averaging about 2-3.

Hey guys,
So I was wondering about any sorts of dampening mechanisms that could be used. We have been asking for rule clarifications on the national and state website, but neither have replied. What is your take on this: using parachutes to slow the fall and hence lowering the standard deviation of the counted length hence making the drops more accurate. We watched the bungees at nats, and it seems that no one did anything like this, but that surprises me.

TheLeftEye wrote:Hey guys,
So I was wondering about any sorts of dampening mechanisms that could be used. We have been asking for rule clarifications on the national and state website, but neither have replied. What is your take on this: using parachutes to slow the fall and hence lowering the standard deviation of the counted length hence making the drops more accurate. We watched the bungees at nats, and it seems that no one did anything like this, but that surprises me.

probably because that wouldn't be allowed under "self-limiting brake mechanisms"

We actually saw one at an invitational that was gently lowered by a motor with a remote. It won, which would really have p!ssed me off if it were not an invitational.

TheLeftEye wrote:Hey guys,
So I was wondering about any sorts of dampening mechanisms that could be used. We have been asking for rule clarifications on the national and state website, but neither have replied. What is your take on this: using parachutes to slow the fall and hence lowering the standard deviation of the counted length hence making the drops more accurate. We watched the bungees at nats, and it seems that no one did anything like this, but that surprises me.

The idea of parachutes was discussed in last year's Bungee Drop C topic.

asdfqwerzzz2 wrote:

The Architect wrote:What are people interpreting a "Self-limiting-brake" to be? Would this consider things such as parachutes to be brakes?

I believe it's saying that the elastic has to brake itself and go back to its original position by itself. This would include parachutes.

Flavorflav wrote:We actually saw one at an invitational that was gently lowered by a motor with a remote. It won, which would really have p!ssed me off if it were not an invitational.

LOL well that's not allowed XD

chinesesushi wrote:

Flavorflav wrote:We actually saw one at an invitational that was gently lowered by a motor with a remote. It won, which would really have p!ssed me off if it were not an invitational.

LOL well that's not allowed XD

It's clearly a spirit of the problem violation, however it's not explicitly prohibited in the rules, so perhaps a FAQ is in order.