[普物] 作業
int 1/2*( R-(r^2-z^2)^1/2)^2 + ( R+(r^2-z^2)^1/2)^2 ) dm (P.253 table(b)
= int (R^2+(r^2-z^2))*d'*4*pi*R*(r^2-z^2)^1/2*dz
(d'=density) (dm=pi*( R+(r^2-z^2)^1/2)^2-(R-(r^2-z^2)^1/2)^2)*dz
=4*pi*R*d'*( int R^2*(r^2-z^2)^1/2*dz + int (r^2-z^2)^3/2*dz
=4*pi*R*d'*( R^2*pi*r^2 /2 + 3/8*r^4*pi )
(int (r^2-z^2)^1/2*dz =half area of a circle
(int (r^2-z^2)^3/2*z=-u/8*(2*u^2-5*a^2)(a^2-u^2)^1/2+3/8*a^4*arcsin(u/a)+c
=(2*pi*R*pi*r^2)*d'*R^2+(2*pi*R*pi*r^2)*d'*3/4*r^2
=M*R^2+3/4*Mr^2
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