 Open Journal of Applied Sciences, 2013, 3, 65-69 doi:10.4236/ojapps.2013.31B1013 Published Online April 2013 (http://www.scirp.org/journal/ojapps) A Geometrical Theorem about the Static Equilibrium of a Common-point-force System and its Application Guo-quan Zhou Department of Physics, Wuhan University, Wuhan, Hubei, China Email: zgq @whu.edu.cn Received 2013 ABSTRACT A geometrical theorem for the static equilibrium of a common-point-force system has been proven by means of vir-tual-work principle: The equilibrium point of a common-point force system has a minimal weighted distance summa-tion to every fixed point arbitrarily given on each force line with a weighing factor proportional to corresponding force value. Especially the mechanical simulating technique for its inverse problem has been realized by means of pulley block. The conclusions for the inverse problem derived from mechanic method are in accordance with that given by the pure mathematical method, and the self-consistence of the theorem and its inverse problem has been demonstrated. Some application examples in engineering, economy and mathematics have been discussed, especially the possible ap-plication in the research of molecular structure, has also been predicted. Keywords: Spatial Common-point-force System; Static Equilibrium; Minimum Theorem；Principle of Virtual Work; Depot Problem; Transportation 1. Arising of the Problem and its Mathematical Model The so-called depot problem [1-5], one of the edging subject in the field of transportation science and modern architectural economy, is related directly to the economic efficiency and the industrial and commercial interests. The suitably chosen “depot” can save much man power and working time, decrease unnecessary loss of material resources. Recent years, with the fast development of mathematics, computer technology and operational re-search, some methods have been found to solve the multi-depot vehicle routing problem, such as the Integer Programming Model combined with the Hybrid Genetic Algorithm, the Nearest Neighbor Heuristic Method, and the Tabu Search Method based on Adaptive Memory Principle[1-5], and so on. One of the basic problems is just like the following example. Provided 12P,P,, PN are N disperse working sites for vehicle transportation, the possible location of the main traffic station P has mean i times of to-and-fro transportation between P and () each month, then which location P can have a minimal weighted distance summation to every working site and minimize the total fuel consumption ? nPi1, 2,,iNThis question can be expressed as a mathematical question: to look for a point P which can minimize the double weighted distance summation: 111222 2() NiiiNNSnPPnPPnPPnPP (1) From the viewpoint of fuzzy mathematics, number i’s---- the weighing factors appeared in above ques-tions are average values with respect to time and needn’t to be integers, they can be any positive real numbers. nA solution to such a problem is found to be related to a mechanical model—the static equilibrium of a com-mon-point-force system (CPF system for brevity), which is based on the principle of minimal potential for the static equilibrium of a conservative mechanical system in the gravitational field. Another equivalent method of virtual-work principle under ideal constraint is an alter-native solution to the inverse case of above problem. In the following section, we will throw light on such a fact that static equilibrium method of CPF system can solve the problem of least weighted distance summation for both two and three dimensional cases. It can also be used to deal with cases involved in not only the straight path but also the blocked or curved path. 2. A Geometrical Theorem for the Balanced CPF System Based on the principle of virtual work, a geometrical theorem for the static equilibrium of a CPF system can Copyright © 2013 SciRes. OJAppS G. Q. ZHOU 66 be easily proved. Then the correlation between the theo-rem and above problems will be immediately established after a suitable mechanic model has been constructed. Theorem The equilibrium point of a common-point force system has a minimal summation of weighted dis-tance from every fixed point arbitrarily given on each force line, with a weighing factor proportional to corre-sponding force value. The above theorem means, for N balanced forces Fi) with common point P and given values i respec-tively (, and f is a common factor with force’s dimension) and for N fixed points nf (11, 2,iN,2,Ni in each force line respectively, the equilibrium point P of a CPF system must minimize the weighted distance Pisummation , i.e. for any other point 1NiiPSn PPiP, min11NNii iiiSnPPS nPiP) (2) where satisfy following relation: (1,2,ini N12 12:: ::: :FF F NNnn n (3) By means of virtual-work principle about stable equi-librium of a multi-force system, the above theorem can be easily proven as follows. For an arbitrary virtual displacement r (from 0) of the equilibrium position P and PP riri of each fixed point , suppose  , the vir- (1,2,iPi N))iPPtual work of the resultant force is zero under 1F= FNiiideal constraint, that is 10Fr FrNii (4a) or 10FrNiii (4b) which is duo to following fact (1,2, ,rr iiN (5) On the same time, forces (1,2,)iFiNN are all centric forces that respectively direct at fixed point (), so we have iP1, 2,i()rFiiiinf r (6) where Fiinf, f is the common factor with dimension of force and the dimensionless positive real constant; inriir is the unit vector along with or the direction of iPPforce iF: according to the property of centric force[7-9], its work can be express as (Fr )iiiinf r (7) Then the condition (2) can be written as: 1()Niiinf r0 (8) 10Niiinr (9) This is just the necessary and in fact also the sufficient condition for weighted distance summation 11NNiii iiinrnPP to attain its minimum. Then the above theorem get veri-fied. For the special case that (a sufficient 1,Nijjinjnbut not necessary condition), the CPF system can’t attain equilibrium state by itself, then point P will move to cer-tain a point i and attain equilibrium by means of forced constraint reaction at . Thus, point is just PiPiPthe wanted point P to minimize the sum 1NiiinPP. The theorem (2) is actually the mathematical criterion for the static equilibrium of the CPF system. On the other hand, this geometry theorem also clearly reflects the geometric property of the CPF system having attained equilibrium state, and what is more, reflects the spatial property of the concerted fields of forces. On the other hand, a stable static equilibrium of the CPF system corresponds to the case of the minimal sum, and an unstable static equilibrium corresponds to the case of the maximal sum. 3. A Mechanical Model and Solution to the Inverse Problem There had ever been some discussion in history about the so-called depot problem or factory-location problem similar to question in section 1. It is easily found to be the inverse problem of the above theorem, and can be solved by mechanic technique. Reference  had given two mechanic models to solve such a geometrical prob-lem in the two-dimensional case, one is the model of Figure 1. The balanced common-point-force system. Copyright © 2013 SciRes. OJAppS G. Q. ZHOU 67surhe face tension of liquid cake films, the other is tmodel of ropes with suspended weights passing through smooth holes on a frictionless table. But in three-dimen-sional case, both models are invalid. In order to find a feasible technique to solve the problem in the three-di-mensional case, we can always proportionately minify the real distribution of N given point 12,,NPP P to a model of miniature distribution 12,, NPP bora-tory, shown as Figure 2, then instnless fixed pulleys at each point (1,2,)iNPiN . Each rope with length of L is conneto a suspended weight iG of mass in at one end and to the common point P ate other endand passes by the fixed pulley at iP respectively. Note that each fixed pulley has a defi- position and adjustable free axis. Obviously the whole CPF system is a pure gravitation-doing-work system un-der ideal constraint, i.e. a conservative system. When the system attains static equilibrium, the whole system will possess the minimal total gravitation potential energy with respect to any given horizontal plane P in la frictioy all Nvelcted respecti, thnite, which is chosen to be the zero plane of potential energ(shown as Figure 2). If each of the point of 12,, y NPP P has the height of 12,,Nhh h respectively w to plane ith respect, then each particle iG has its gravitation potential ergy of ()iiiing hPGspectively (1, 2,in). Then the total gravenergy )en reitation potential 1(iiiiing hPG Nmust get the minimal for the equilibrium state, and i1iiing PGN must attain maximum because 1iiingh (Nis a constant. Duo t, then )o ii iPGL PP1inL PP st attain its Ni mui maxii ition is that the whole common-point-force symust be themal, and 1NinPiPminimum. The condstem attains its static equilibrium. And the equilibrium position P is just the wanted point to make the weighted distance summation 1iinPPN get its minimum. iIt should be pointed out once more that the weighted distance summation may attain its minimum at the boundary point such as (1,2, )iPi N- when the equilibrium state can only beans of forced constraint reaction at iP, then the point iP is just the wanted point satisfyinge minimum conditin. Here, we give a sufficient but not necessary condition, that is: When N, point P is just the wanted attained by me tho1, ijjjinnes the summai1iiiThe condition is that the whole coorce system attains its static equilibrium. And the equilibrium position P is just the wanted point to make the weighted distance summation 1NiiinPP get its minimum. point that maktion the minimal. mon-point-fout oncghted distance summation may attain its minimum at the boPPmNSnIt should be pointed e more that the weiundary point such as (1,2,)iPi N- when the equilibrium state can only be attained by means of forced constraint reaction at iP, ti is just the wanted point satisfying the minimum condition. Here, we give a sufficient but noecessary conditionthat is: When 1, hen the point Pt n, Nijjjinn, point iP is just the wanted point that mists had everthis mechanical analogue of the depot problem in the 2-Concrete Conclusion for the m, the abovdirectly lead to some inter- method: aketion NSnPP the minimal. For many years econom misinterpreted s the summa1iiidimensional case, and mistakenly thought the center of mass cP to be the wanted point to make the weighted distance summation minimal. Actually, cP, the center of mass, leads to the minimal weighted square distance summation and not the minimal weighted distance sum-mation. 4. The Three-Force System For the simple case such as 3-CPF or 4-CPF systee mechanical model will esting and valuable conclusions. In field of geometry, we can prove following conclu-sion by means of pure mathematicalFor any triangle 123PPP with its three inner angles not bigger than 120, there must be a point P within the triangle to minimize the distance summation 123PPPPPP when and only when 12PPP23 3123PPPPPP. Figure 2. The mechanical model. Copyright © 2013 SciRes. OJAppS G. Q. ZHOU 68 The same conclusion can be drawn by means of static equilibrium of common-point system, and what is more, the conclusion can be extended further. For 123123, ::::FFFNnhe sufficient and necessary co3nn, it is easy to deduce tndition for the equilibrium of 3-CPF system, that is ①, the three forces are coplanar. ②, following equation must be satisfied: 312233112sin(, ) sin(,) sin(,)FFFFFFFFF (10) Expression . Here (10) is just the Lami’s theorem(F, )Fij expresses the angle between two vectors Fi and Fj (not larger than ). According the theorem (2), when expression (10) is satisfied he wanted point to make the weighted distance summation 11223nPP nPPnP, corresponding point P is just t3mP inimal. Then it means 312233112sin(,)sin(,)sin( ,)FFFF FFnnn (11) Considering 1223 31(,)(, )(,) 2FFFF FF  , we make formation 1π(a trans32,)231312π(,)π(,)FFFFFF (12) Then 123 , and 123,, can be viewed as the three inner angles of certain a triangle. According the cosine theorem, following identity holds to the sine theorem and222222sinsin sincos 2sin sin2 (.. (11))  jkijkijknnncfHere can be permutated in turn within 1,2,3. Then ijkn(13) n,,ijk 222j( )πarccos 2FF jkijk k (14) So when the CPF system attains static equilibrium, the open angle of point P with respect to ,,PPPPPP respectively cannnnn，n be calculated according to (14). Specially when according to (14) 1223 31123nnnn1233, then 122331 2π, )(,)(,)3F FF FF (15) Meanw123()SnPPPPPP just coincident with that gmethod. This is not onlyexouthat makes 1the minimal. This conclusion is iven by pure g one of the special application ) and the mepointing eometrical amples of CPF system, but also a convincing geomet-rical side proof for the above theorem (2-chanical technique. By the way, it is worthy of t, when n=3 and is the needed point 12nnn, 231P3123P n PPnPnP(here 10P be the minimal PP ), or min212313PPthod, we canP. n eaSne mePsily extend our conclusions for case of to the case 5. Discussion of a sere principle of virtual woential energy are s of t littom above because of the different ex-ost intereen in civil engineering and in-duBy means of the sam3n of 3n. Extension of Application Fields and Some The theorem (2) and the mechanical model given by this paper can be used to deal with both the coplanar CPF case and spatial CPF case. If we replace the pulley block at points of 12,nPP P by n smooth and frictionless small hole on the surphe crust, an ideal con-straint case, thrk and the method of minimal total pottill valid. As a re-sufacelt, many geometric questions on the surface of a sphere can be solved at once. Just as what has been mentioned before, theorem (2) reflects the spatial propertyhegravitation fields near to surface of earth. On a largespace in the gravitation field, the conclusion must have ale difference frpression of gravitation potential energy, and there must be also some spatial distortion effects resulting from the gravitation of the earth. One of the msting applications of the theorem and the mechanical model may be research of molecular structure. A stable molecular structure of multi-atom molecule must obey a geometrical constraint condition. Every chemical bond has definite bond strength and a bond length under static equilibrium state, each bond strength (in terms of force) equals to the gradient of the potential energy along with corresponding chemical bond. Then there must be a constraint condition among each chemical bond in view of theorem (2) under the stable equilibrium. Other possible application fields of this paper include most of the problem arisstry design such as the factory location; the city drain-age system; the choice of the trifurcation point of traffic roads. In a word, the mechanical model of the static equi-librium of CPF system can solve most of the preliminary design that concerned problem of the minimal weighted distance summation. Its application can spare much man-power and material resource, decrease social operation cost and promote the economy efficiency. Meanwhile, it can work as a powerful tool to study some problems of mathematics, mechanics and even the gravitation field. (Fhile the corresponding point P will make Copyright © 2013 SciRes. OJAppS G. Q. ZHOU Copyright © 2013 SciRes. OJAppS 69ledgements 6. AcknowThis work was supported by the National Natural Sci-ence Foundation of China under Grant No.10775105. REFERENCES  H. William, T. S. H. George, J. Ping and C. W. L. 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