Simple Nature - Light and Matter

and since sin θ dθ occurs in the integral, the easiest path is to substitutefor it, and get everything in terms of r and dr:U = − 2πGρbhms= − 4πGρb2 hms= − GMms∫ s+bThis was all under the assumption that mass m was on the outsideof the shell. To complete the proof, we consider the case where it’sinside. In this case, the only change is that the limits of integrationare different:U = − 2πGρbhms= −4πGρbhm= − GMmbs−b∫ b+sb−sdrdrk / The gravitational energyof a mass m at a distance s fromthe center of a hollow sphericalshell of mass.The two results are equal at the surface of the sphere, s = b,so the constant-energy part joins continuously onto the 1/s part,and the effect is to chop off the steepest part of the graph that wewould have had if the whole mass M had been concentrated at itscenter. Dropping a mass m from A to B in figure k releases thesame amount of energy as if mass M had been concentrated at itscenter, but there is no release of gravitational energy at all whenmoving between two interior points like C and D. In other words,the internal gravitational field is zero. Moving from C to D bringsmass m farther away from the nearby side of the shell, but closerto the far side, and the cancellation between these two effects turnsout to be perfect. Although the gravitational field has to be zeroat the center due to symmetry, it’s much more surprising that itcancels out perfectly in the whole interior region; this is a specialmathematical characteristic of a 1/r interaction like gravity.Newton’s apple example 17Over a period of 27.3 days, the moon travels the circumferenceof its orbit, so using data from Appendix 5, we can calculate itsspeed, and solve the circular orbit condition to determine thestrength of the earth’s gravitational field at the moon’s distancefrom the earth, g = v 2 /r = 2.72 × 10 −3 m/s 2 , which is 3600times smaller than the gravitational field at the earth’s surface.The center-to-center distance from the moon to the earth is 60times greater than the radius of the earth. The earth is, to a verygood approximation, a sphere made up of concentric shells, eachwith uniform density, so the shell theorem tells us that its externalgravitational field is the same as if all its mass was concentrated104 Chapter 2 Conservation of Energy