10 Consercation of Momentum 10-1 Newton's Third Law On the basis of Newton's second law of motion,which gives the relation 10-1 Newton's Third Law between the acceleration of any body and the force acting on it,any problem in 10-2 Conservation of momentum mechanics can be solved in principle.For example,to determine the motion of a few particles,one can use the numerical method developed in the preceding chapter. 10-3 Momentum is conserved! But there are good reasons to make a further study of Newton's laws.First,there 10-4 Momentum and energy are quite simple cases of motion which can be analyzed not only by numerical methods,but also by direct mathematical analysis.For example,although we 10-5 Relativistic momentum know that the acceleration of a falling body is 32 ft/sec2,and from this fact could calculate the motion by numerical methods,it is much easier and more satisfactory to analyze the motion and find the general solution,s=so+vot+1612.In the same way,although we can work out the positions of a harmonic oscillator by numerical methods,it is also possible to show analytically that the general solution is a simple cosine function of t,and so it is unnecessary to go to all that arithmetical trouble when there is a simple and more accurate way to get the result.In the same manner,although the motion of one body around the sun,determined by gravitation,can be calculated point by point by the numerical methods of Chapter 9,which show the general shape of the orbit,it is nice also to get the exact shape, which analysis reveals as a perfect ellipse. Unfortunately,there are really very few problems which can be solved exactly by analysis.In the case of the harmonic oscillator,for example,if the spring force is not proportional to the displacement,but is something more complicated,one must fall back on the numerical method.Or if there are two bodies going around the sun,so that the total number of bodies is three,then analysis cannot produce a simple formula for the motion,and in practice the problem must be done numeri- cally.That is the famous three-body problem,which so long challenged human powers of analysis;it is very interesting how long it took people to appreciate the fact that perhaps the powers of mathematical analysis were limited and it might be necessary to use the numerical methods.Today an enormous number of problems that cannot be done analytically are solved by numerical methods,and the old three-body problem,which was supposed to be so difficult,is solved as a matter of routine in exactly the same manner that was described in the preceding chapter,namely,by doing enough arithmetic.However,there are also situations where both methods fail:the simple problems we can do by analysis,and the moderately difficult problems by numerical,arithmetical methods,but the very complicated problems we cannot do by either method.A complicated problem is, for example,the collision of two automobiles,or even the motion of the molecules of a gas.There are countless particles in a cubic millimeter of gas,and it would be ridiculous to try to make calculations with so many variables (about 1017- a hundred million billion).Anything like the motion of the molecules or atoms of a gas or a block or iron,or the motion of the stars in a globular cluster,instead of just two or three planets going around the sun-such problems we cannot do directly,so we have to seek other means. In the situations in which we cannot follow details,we need to know some general properties,that is,general theorems or principles which are consequences of Newton's laws.One of these is the principle of conservation of energy,which was discussed in Chapter 4.Another is the principle of conservation of momentum, the subject of this chapter.Another reason for studying mechanics further is that there are certain patterns of motion that are repeated in many different circum- 10-1