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The physics behind the rubber band test?
- Thread starter KuKu
- Start date
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- Nov 16, 1998
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- 1,345
Call me stupid, but I don't get the point, actually, I don't get it period
Understand my signature!
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" Knife Collectors Are Sharp People - Most of the time that is! "
Understand my signature!
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" Knife Collectors Are Sharp People - Most of the time that is! "
- Joined
- Oct 7, 1998
- Messages
- 1,838
DannyC; you are correct, many people would guess momentum. Remember, however, that momentum (mass x velocity) is directly porportionate to the velocity.
Energy, however (mass x velocity squared /2) is proportional to the square of velocity; this geometric progression is what allows the rapidly moving blade to cut through the rubber band while its' intertia holds it in place, before its' elastic tension can accelerate it out of the way.
Remember that any edged object works because it concentrates force in a very small area, over a very small TIME.
Note, if you please, that even if the tension of the rubber band is increased, the band still must start accelerating from a standstill, thus obviating most of the advantage of the increased tension.
Actually, I have no idea what in the heck a rubber band test is. However, if KuKu can cut a stretched rubber band into six pieces, he must work at Benehana! Walt
Energy, however (mass x velocity squared /2) is proportional to the square of velocity; this geometric progression is what allows the rapidly moving blade to cut through the rubber band while its' intertia holds it in place, before its' elastic tension can accelerate it out of the way.
Remember that any edged object works because it concentrates force in a very small area, over a very small TIME.
Note, if you please, that even if the tension of the rubber band is increased, the band still must start accelerating from a standstill, thus obviating most of the advantage of the increased tension.
Actually, I have no idea what in the heck a rubber band test is. However, if KuKu can cut a stretched rubber band into six pieces, he must work at Benehana! Walt
Here's what the rubber band test is:
http://www.bladeforums.com/ubb/Forum32/HTML/001673.html .
One way to find out what causes the rubber band to be cut into more than one piece would be to shoot the sequence using high speed film and play it back in slow motion. That would be interesting.
-dzig
http://www.bladeforums.com/ubb/Forum32/HTML/001673.html .
One way to find out what causes the rubber band to be cut into more than one piece would be to shoot the sequence using high speed film and play it back in slow motion. That would be interesting.
-dzig
UffDa
Gold Member
- Joined
- Sep 11, 1999
- Messages
- 42,596
SON OF A GUN! IT WORKS!
It had to play with the edge a bit to get it to work. The edge on my Wegner Jr. had a polished hair popping edge, but it wouldn't cut the rubber band. I resharpened it on a medium DMT file and deburred in on a fine crock stick. It still shaved, but not as nice as the polished edge.
The first rubber band was cut in 3 pieces, one large chunk and two small, 1/4", pieces.
The main piece had 4 pretty good cuts that didn't go all the way through.
The second rubber band was cut into 4 pieces. 1 large piece and 3 small pieces of varying sizes. The were also several small cuts in the main chunk. I tried it several more times with the same results.
My guess it that when you let go of the rubber band it folds over on itself just before it hits the edge. It happens to fast to see. Very interesting.
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Do not lead me for I will not follow.
Do not follow me for I will not lead.
Do not walk by my side either.
Just leave me the hell alone.
[This message has been edited by UffDa (edited 05-11-2000).]
It had to play with the edge a bit to get it to work. The edge on my Wegner Jr. had a polished hair popping edge, but it wouldn't cut the rubber band. I resharpened it on a medium DMT file and deburred in on a fine crock stick. It still shaved, but not as nice as the polished edge.
The first rubber band was cut in 3 pieces, one large chunk and two small, 1/4", pieces.
The main piece had 4 pretty good cuts that didn't go all the way through.
The second rubber band was cut into 4 pieces. 1 large piece and 3 small pieces of varying sizes. The were also several small cuts in the main chunk. I tried it several more times with the same results.
My guess it that when you let go of the rubber band it folds over on itself just before it hits the edge. It happens to fast to see. Very interesting.
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Do not lead me for I will not follow.
Do not follow me for I will not lead.
Do not walk by my side either.
Just leave me the hell alone.
[This message has been edited by UffDa (edited 05-11-2000).]
I think UffDa is right. When you let go of the rubber band (think in slo-mo) it begins to wrinkle up as it contracts, like an accordion. It makes little horizontal peaks and valleys. If the edge is sharp enough, it just lops off the peaks as they go by. How's that for a scientific explaination? Anybody buy it?
um...nope! sorry
The rubber band is already under tension and would only return to its natural size. At one point it will be in its natural size and the momentum will carry it into the blade. So no compression valleys or peaks. I think,..,,,,,,,Do you guys think a web cam is slow/fast enough to catch it on film?...
The rubber band is already under tension and would only return to its natural size. At one point it will be in its natural size and the momentum will carry it into the blade. So no compression valleys or peaks. I think,..,,,,,,,Do you guys think a web cam is slow/fast enough to catch it on film?...
I don't know if a webcam would keep the resolution, but please try! I am dying to know how it happened. I still think I'm right(mabey!). I just tried an experiment. If we loop the rubber band as instructed and stretch it back, lets call the part of the band that is touching the spine of the blade "A" and the part that we are grasping "B". As soon as we let go, Part A does not jumb off the spine of the blade. Intertia keeps it there until the rubber is completely returned to its original size and begins to push "A" away, or when "B" actually meets "A" by being sliced in two and passing by. The two ends of "B" travel past "A" and eventually pull "A" off the spine of the blade.
At least that's what I thought. So I tried it in slo-mo. I took a band, and my knife and rigged it as above. Instead of letting go I slowly brought "B" to "A". You're right, the band does not compress and zig,zag. It pushes "A" away before it does that. Hoever, the area near "B" does bend in random patterns as it travels toward the blade. Often the part that touched the blade first was a full centimeter from "B". If the blade cut through this point before "B", then "B" came right behind it and cut again, you would cut an individual chunk out of the band. If this "wrinkle" effect happened more than once, you get more pieces.
I know it's a thin story, but I'm stickin' to it. Somebody please film this!
At least that's what I thought. So I tried it in slo-mo. I took a band, and my knife and rigged it as above. Instead of letting go I slowly brought "B" to "A". You're right, the band does not compress and zig,zag. It pushes "A" away before it does that. Hoever, the area near "B" does bend in random patterns as it travels toward the blade. Often the part that touched the blade first was a full centimeter from "B". If the blade cut through this point before "B", then "B" came right behind it and cut again, you would cut an individual chunk out of the band. If this "wrinkle" effect happened more than once, you get more pieces.
I know it's a thin story, but I'm stickin' to it. Somebody please film this!
Do more cuts/pieces equate to a sharper knife? I tried this yesterday with my large Sebenza that had just come back from CRK where it was sharpened. The large Sebenza consistently produced one cut of the rubber band. I tried the same test with my small Sebenza which I thought was sharp but had not been sharpened in a while. No Cut. I resharpened the small Sebenza on a 204 and tried again. Consistently got multiple cuts/pieces with a high of FIVE!
-Steven
-Steven
UffDa
Gold Member
- Joined
- Sep 11, 1999
- Messages
- 42,596
Try this. Deside where you are going to hold the rubber band and mark that as the center. Mark the left and right sides of the rubberband so you can tell which side the pieces came from. You might do this experiment over some newspaper so you can find the pieces.
I just did what I described and here's what I got.
The main section of the band has two small cuts on the left side. The right side was cut into 5 small pieces ranging from 1/4" to 5/8". There are also some small cuts on what's left of the right side. The only conclusion I can draw from this is that as the rubber band snaps back into it's original shape it goes through some pretty wild girations and folds over on itself. Without highspeed photos to prove otherwise, there is no other reasonable explanation.
Most of the rubber bands that I have tried are not perfectly round at rest. If they were perfectly round the results might be different. I'll run this by my son, the physicist, the next time I see him.
[This message has been edited by UffDa (edited 05-11-2000).]
I just did what I described and here's what I got.
The main section of the band has two small cuts on the left side. The right side was cut into 5 small pieces ranging from 1/4" to 5/8". There are also some small cuts on what's left of the right side. The only conclusion I can draw from this is that as the rubber band snaps back into it's original shape it goes through some pretty wild girations and folds over on itself. Without highspeed photos to prove otherwise, there is no other reasonable explanation.
Most of the rubber bands that I have tried are not perfectly round at rest. If they were perfectly round the results might be different. I'll run this by my son, the physicist, the next time I see him.
[This message has been edited by UffDa (edited 05-11-2000).]
UffDa,
I believe that you probably have the answer to the multiple cut results of the rubber band test. The rubber band is not under the same tension along the entire length. It probably has regions of varying compliance, as well. The main points of attachment, blade spine and fingers, are probably not in perfect allignment. They also vary in contact area size, and the release is probably not perfect. With all these variables considered, the tendency of the rubber to rebound to the original configuration should cause varying degrees of undulation and result in the generation of waves. The folding that you suggest would then result and the multiple cuts would occur. Of course it could also be the result of some ancient rubber tree god trying to get even with us.
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Dave
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Attention: Some assembly may be required. Batteries not included.
I believe that you probably have the answer to the multiple cut results of the rubber band test. The rubber band is not under the same tension along the entire length. It probably has regions of varying compliance, as well. The main points of attachment, blade spine and fingers, are probably not in perfect allignment. They also vary in contact area size, and the release is probably not perfect. With all these variables considered, the tendency of the rubber to rebound to the original configuration should cause varying degrees of undulation and result in the generation of waves. The folding that you suggest would then result and the multiple cuts would occur. Of course it could also be the result of some ancient rubber tree god trying to get even with us.
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Dave
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Attention: Some assembly may be required. Batteries not included.