20150503, 02:26 PM  #21 
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回覆: COM
The first version of my definition of COM : "In physics, the center of mass is a point in space where the weighted relative position of the distributed mass sums to zero."
So turned out it is too technical for someone to understand. I already answered it is a weighted function and in this "weight function" calculation, the influence is the mass. Again, that's still too technical. Okay, okay, let's put it in a even less technical way then. If you glue two balls (ball A and ball B, each has perfectly even mass density) with same radius (radius = r) together and create one object. I call this object has 2 distributed mass located, each located at the center of ball A and B. 1) If the mass of ball A and B are exactly equal. (note: I didn't say "weight" the noun, I said "mass"). Now both ball has the equal amount of influence (same mass) in the calculation. The COM of this glued object is located exactly half way between the center of Ball A and Ball B. This is where the sum of "relative position of ball center A" and "relative position of ball center B" under equal influence (same mass) equal to zero. 2) if the mass of ball A is twice of ball B (note: I didn't say "weight" the noun, I said "mass"). In the same calculation of COM, ball A will have twice the influence compare to ball B (mass of A is 2 times of mass of B). The COM of this glued object is located in the line connecting ball center A and B, but closer to center of ball A. It is located at along the line jointing the center of ball A and B with 2/3 r (radius = r) away from ball A center. It is also 4/3 r away from ball B center on the exact opposite direction. At this COM location: 1) relative position of ball A center is 2/3 r 2) relative position of ball B center is 4/3 r 3) influence of ball A is 2 4) influence of ball B is 1 "Weighted sum" of the relative position = relative position of ball A * influence of ball A + relative position of ball B * influence of ball B = 2/3 r * 2 + (4/3 r) * 1 = 4/3 r  4/3 r = 0 [Nowhere above involves any gravity, force, etc. Just mass in space.] [I didn't said anything like put the “weight” into the “mass”. I don't know how one could create that term and got confused. I cannot answer this one question at all.] [I agree with “Arithmetic” is only dealing with the techniques of the calculation. Weighted sum, weighted average is just like +, , *, /. It is a technique of calculation. There is nothing special about it and it is just another "arithmetic". Everything could make use of this kind of calculation. It is wrong for you to say it used by statistic alone. Physics calculation use all kinds of arithmetic in calculation. Weighted average for complex case in COM calculation, like uneven density or odd shaped object needs, to use integration. Is calculating an integral considered as a “math” by “pure” mathematician?] [Again, I'm 100% sure what's I'm talking about here and that's the COM meaning used by most people in this world. If you don't like it, you are welcome to prove me wrong.] [There is nothing wrong for me to say COM = Center of Gravity if it is under an uniform gravity field. Is there anything wrong to say the "length of a rod" is equal to "the height of the rod end when you place the other end vertically on the floor"? When there is no "floor" around, the second part of the measurement is no longer valid. However, that doesn't mean the rod has no length. The rod length doesn't change at all, whether or not there is a floor around. The same for COM, COM doesn't change location whether gravity is involved or not since gravity is not being involved in my definition of COM.] 此篇文章於 20150503 02:30 PM 被 B2L2 編輯。 
20150503, 02:40 PM  #22 
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回覆: COM
Okay, let see why high jumper "move" his Center of Mass outside of his body to obtain good results:
It isn't me who proved you wrong. Mr Dick Fosbury did. 
20150503, 11:01 PM  #23  
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“In mathematics and physics, the centroid or geometric center of a twodimensional region is the arithmetic mean ("average") position of all the points in the shape. The definition extends to any object in ndimensional space: its centroid is the mean position of all the points in all of the coordinate directions. Informally, it is the point at which a cardboard cutout of the region could be perfectly balanced on the tip of a pencil, assuming uniform density and a uniform gravitational field. While in geometry the term barycenter is a synonym for "centroid", in physics "barycenter" may also mean the physical center of mass or the center of gravity, depending on the context. The center of mass (and center of gravity in a uniform gravitational field) is the arithmetic mean of all points weighted by the local density or specific weight. If a physical object has uniform density, then its center of mass is the same as the centroid of its shape. In geography, the centroid of a radial projection of a region of the Earth's surface to sea level is known as the region's geographical center.” http://en.wikipedia.org/wiki/Centroid 引用:
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“No, you said COM has nothing to do with gravity, even use the word "again," you liar. “Again, COM has nothing to do with gravity or force. It is mass only. Even in 0G environment, an object still have COM.” http://www.goski.com.tw/forum/showpo...4&postcount=14” :) IS 

20150503, 11:26 PM  #24  
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:) IS 此篇文章於 20150504 12:47 AM 被 taichiskiing 編輯。 

20150505, 03:25 PM  #25  
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Your definition of COM, you said it doesn't exist under 0G. This is WRONG. 引用:
WRONG statement that you made. If I have to correct your statement, I could said "in physics, under a uniform force/gravity field, one could consider it only acts at the centroid/center of mass/center of gravity". This statement said there location is the same under the condition of a uniform force/gravity field. In the Wiki page, it has been repeated a number of time. Why do you think they have to repeat this again and again? The fact is when under a "nonuniform" force/gravity field, the COM location isn't equal to CG. If you want to find out the COM and CG of a really long rod placed vertical on ground, the two position are NOT the same. Do you know why? Gravity is always lower at higher elevation. CG is always a little bit lower than COM if an object is placed on Earth. Reason? The lower part of the object experience a higher force compare to the higher part of the object. Yes, if we are talking a human height object, the different of G could be consider really really small since the height different is too small. Mathematically they aren't the same on planet Earth, they are only very close to each other. I asked about COM of solar system. You said you know the answer, but I really doubt that is correct given all the wrong understand you have. Mind to give out the answer so that I could prove it wrong? 引用:
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 http://www.differencebetween.com/dif...enterofmass/ Quote: "Center of mass is a fixed property which is the average location of the mass of the body. It has nothing to do with gravity." Oh, yes, it even use the same wording as mine : "It has nothing to do with gravity." 此篇文章於 20150505 03:43 PM 被 B2L2 編輯。 

20150505, 03:38 PM  #26  
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From now until you could provide that prove, I would "say" you're wrong. [You claimed the COM is located at the point inside the body closest to the point in the video. Could you explain under which Physics rule that Mr. Dick Fosbury could jump half a meter higher with this style?? Does Mr. Dick Fosbury has a body thickness of 0.5 meter? If not, what energy empower Mr. Dick Fosbury to jump so much higher? Could you explain how Mr. Dick Fosbury could utilize the “imbalance” property of COM (which you claim still located inside his body) to achieve this 0.5 meter gain by using this style of jump only?] 

20150505, 11:13 PM  #27  
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今次偽太極又可能冇言以對，又走佬一段時間。 

20150506, 02:13 AM  #28  
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:) IS 

20150506, 02:20 AM  #29  
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:) IS 

20150506, 04:24 AM  #30  
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此篇文章於 20150506 04:45 AM 被 pku 編輯。 

20150506, 07:08 AM  #31  
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應時應景，我的滑雪是“活”的‧ :) IS 

20150506, 10:42 AM  #32  
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每個人滑雪都是活的，低手到陡坡自然要橫滑減速， Side slip 當然不能避免，不過你 side slip 也控制得不大好，所以滑橫仍是你核心技術，什麼腳隨意轉，還是留在好雪平坦的 trail 做，什麼合附物理，數學，講出來做不到，人家也不能拿你打靶 什麼 line skiing , 見歩行歩，陡時橫滑，平坦時轉快些，一年滑一佰日，滑了那麼多年仍滑成這樣，簡直丟人現眼 此篇文章於 20150506 12:50 PM 被 pku 編輯。 

20150506, 01:17 PM  #33 
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回覆: COM
From now until you could provide that prove, I would "say" you're wrong.

20150506, 09:07 PM  #34 
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僞太極講什麼講不過都要死辯說自己是對，說人家不錯丶這是他幾十年在論壇的作風，
不過他不知道經常詆毀的 whistler group 是人材濟濟，我剛 confirm with Derek, he got two master degree, one in information system, one in Physic. 而且他也是台灣人，也是 CSIA 2 ，更是 CASI 2 不過叫他來對付偽太極，偽太極隨時躱回他的狐狸洞，我便沒有人給我駡 
20150507, 01:01 AM  #35  
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是，也不是，地力老人的energy是源源不絕的，就看你懂不懂得，會不會用而已。
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你是連我“這樣”簡單的滑雪都看不懂，“丟人現眼”你自己。 :) IS 

20150507, 01:03 AM  #36 
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20150507, 01:06 AM  #37  
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“無恥鐵齒”，你說的是你們whistler group的作風，我在這裡“証明”之。
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:) IS 

20150507, 11:02 AM  #38 
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[quote=taichiskiing;30220]小知小技小人的“無恥鐵齒”， thanks for proving it./QUOTE]
Other than BSing everyday, why don't just go to find one article or one person who agree with your theory. How about go ask your old professor who taught you to prove you are right? I don't believe there isn't a single article exist in this world that describe a simple theory. My statement remain the same. Until you could find a prove, your theory remain wrong as I call. 
20150507, 12:17 PM  #39  
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你技術那麼低，你帶雪杖對你是一種負累，如果滑翔機像你滑陡坡那樣，幾秒就墮機，無恥老人，你省點吧 你滑雪那麼簡單，一看就明，晨運亞伯遊山，你不說你是高手，就不會丟人現眼 

20150508, 01:25 AM  #40  
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:) IS 
