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joiemoie wrote: At a certain strain, hooke's law breaks. This is roughly at 100% strain. However, fully elastic bungees have a very low strain and always remain relatively constant. At nationals, strain for the fully elastic bungee can go up to 74%, while partially elastic bungees can go past 500%. It will have deviated from hooke's law a long time ago.

Wow...
Just realized that this was the wording that I was looking for back when we were having the discussion about partially elastic vs. fully elastic--I just couldn't figure out how to word it (and couldn't remember the word "Strain" )

blindmewithscience wrote:1-It wears out slower than cords with a non-elastic portion, as it slows it down over a longer period of time and a longer distance.

If you guys do decide to work with a fully elastic cord, one problem that you'll soon come across is that when testing, you stretch the bottom part of the cord much more often than higher parts of the cord, and so the lower part of your cord becomes more elastic than the top (to the point where it deviates from your equation enough to hit the ground). To counter this, find your modulus of elasticity, do ONLY a few trials (under 5) to verify the accuracy, and then you're good. At the competition, do just one more test of your modulus of elasticity right before impound, and use that value for the event.

I am running this event at a Regional Tournament next week - I've never run it before (I've been involved with SO for over 20 years so I have some experience with how events run). Any words of wisdom?

SOCoach wrote:I am running this event at a Regional Tournament next week - I've never run it before (I've been involved with SO for over 20 years so I have some experience with how events run). Any words of wisdom?

To be fair to all teams, please be sure to abide by the time limit for how long teams can prep and run twice. Make sure to perform the elasticity test either during impound or right before competitors begin testing. I would also recommend setting up a bar clamp where the event will be held so competitors can clamp their cords to the bar with a spring clamp; that way, you will not have to hold the cord for them, and they will have more control over the length of their cords.

Hi there everyone! Just thought I'd check in with some things I figured out only a few days before the competition, and how my state competition was.
So, the equation doesn't work with masses at the extremes--and I can't believe I didn't realize this at all before. At low masses, the weight of the cord can't be ignored, especially with cords like my team's (at 25-some grams per meter), and so the cord stretches less than expected. At high masses, the cord over-stretches by the equation--I haven't figured out the why for this one.
At our competition, they decided to make the drop height be the top of the mass-and so our team basically guesstimated or drop length. Our first drop was about 8 inches off (we played it conservative), and the second about 4, which was good enough to be 3rd place. Oh well, no competition can ever be perfect.
WHenever I have time, I'm going to try and figure out a new equation that can take into account cord mass-I'm not entirely sure if this will fix the equation though...
Thanks for all of your help on these forums, guys. I'll check in every now and then to see if I can help any of you with anything.
Thanks again!

blindmewithscience wrote:Hi there everyone! Just thought I'd check in with some things I figured out only a few days before the competition, and how my state competition was.
So, the equation doesn't work with masses at the extremes--and I can't believe I didn't realize this at all before. At low masses, the weight of the cord can't be ignored, especially with cords like my team's (at 25-some grams per meter), and so the cord stretches less than expected. At high masses, the cord over-stretches by the equation--I haven't figured out the why for this one.
At our competition, they decided to make the drop height be the top of the mass-and so our team basically guesstimated or drop length. Our first drop was about 8 inches off (we played it conservative), and the second about 4, which was good enough to be 3rd place. Oh well, no competition can ever be perfect.
WHenever I have time, I'm going to try and figure out a new equation that can take into account cord mass-I'm not entirely sure if this will fix the equation though...
Thanks for all of your help on these forums, guys. I'll check in every now and then to see if I can help any of you with anything.
Thanks again!

Question: What equation did you use? Fully elastic or partially elastic? Also, what material for the elastic part?

blindmewithscience wrote:Hi there everyone! Just thought I'd check in with some things I figured out only a few days before the competition, and how my state competition was.
So, the equation doesn't work with masses at the extremes--and I can't believe I didn't realize this at all before. At low masses, the weight of the cord can't be ignored, especially with cords like my team's (at 25-some grams per meter), and so the cord stretches less than expected. At high masses, the cord over-stretches by the equation--I haven't figured out the why for this one.
At our competition, they decided to make the drop height be the top of the mass-and so our team basically guesstimated or drop length. Our first drop was about 8 inches off (we played it conservative), and the second about 4, which was good enough to be 3rd place. Oh well, no competition can ever be perfect.
WHenever I have time, I'm going to try and figure out a new equation that can take into account cord mass-I'm not entirely sure if this will fix the equation though...
Thanks for all of your help on these forums, guys. I'll check in every now and then to see if I can help any of you with anything.
Thanks again!

If I may, what equation did you use ? ;o We're having trouble coming up with a working equation that gives us consistent results. Also, what were the drop masses/heights given ?

Ok I derived a new equation for fully elastic bungee considering the fact that the bungee cord isn't massless. It makes a big difference.
a = length of unstretched bungee
b = length of stretch
p = mass of bungee per meter length
lambda:


also for the partially elastic bungee:

zerasaw wrote:Yeah I was going to do a quadratic but I plotted them and it was pretty dang planar. I was able to derive the equation by taking the differential vectors of 2 points on the plane and calculating their cross product and substituting into the general equation of a plane.
6th place at regionals beating a Mentor and Solon team; they completely screwed up the height and weight of the bottle. I had the equation to 1cm accuracy the day before, plugged in the numbers and 40cm off the ground...but if 28cm gets you 6th place then you know something's wrong with the event.

Yeah i heard Mentor just kinda guesses in the bungee drop event. And now they are going to Nationals. That's Crazy...

NotFromMentor wrote:

zerasaw wrote:Yeah I was going to do a quadratic but I plotted them and it was pretty dang planar. I was able to derive the equation by taking the differential vectors of 2 points on the plane and calculating their cross product and substituting into the general equation of a plane.
6th place at regionals beating a Mentor and Solon team; they completely screwed up the height and weight of the bottle. I had the equation to 1cm accuracy the day before, plugged in the numbers and 40cm off the ground...but if 28cm gets you 6th place then you know something's wrong with the event.

Yeah i heard Mentor just kinda guesses in the bungee drop event. And now they are going to Nationals. That's Crazy...

The person doing Bungee Drop for Mentor should post in the Unofficial Rankings threads.

I used the fully elastic equation with our cord. Haven't gotten the opportunity to test if it matches the actual cord, though. I'll update you guys if I do get that chance.