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1、10 Conservation of Momentum 10-1 Newtons Third Law On the basis of Newtons second law of motion, which gives the relation between the acceleration of any body and the force acting on it, any problem in mechanics can be solved in pnnciple. For example, to determine the motion of a few particles, one
2、can use the numerical method developed in the preceding chapter. But there are good reasons to make a further study of Newtons laws. First, there 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 k
3、now 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 = s0 + v0t + 16t2 In the same way, although we can work out the positions of a
4、 harmonic oscillator by numerical methods, it is also possible to show analytically that the general solution is a simple cosine function oft, 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
5、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 ve
6、ry 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, s
7、o 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 t
8、o 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
9、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 n
10、umerical, 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 ndiculou
11、s 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 c
12、annot 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 Newtons laws. One of these is the principle of conservation of energy, which was discusse
13、d 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 10-1 Newtons Third Law 10-2 Conservation of momentum 10-3 Mome
14、ntum is conserved! 10-4 Momentum and energy 10-5 Relativistic momentum stances, so it is good to study these patterns in one particular circumstance. For example, we shall study collisions; different kinds of collisions have much in common. In the flow of fluids, it does not make much difference wha
15、t the flmd is, the laws of the flow are similar. Other problems that we shall study are vibrations and oscillations and, in particular, the peculiar phenomena of mechanical waves- sound, vibrations of rods, and so on. In our discussion of Newtons laws it was explained that these laws are a kind of p
16、rogram that says “Pay attention to the forces,“ and that Newton told us only two things about the nature of forces. In the case of gravitation, he gave us the complete law of the force. In the case of the very complicated forces between atoms, he was not aware of the right laws for the forces; however, he discovered one rule, one general property of forces, which is expressed in his Third Law, and that is the total knowledge that Ne
