大物：如何证明
取如图面积元dS
dS=rdrdθ
dm=mdS/π(R2?-R1?)=[m/π(R2?-R1?)]rdrdθ
则 ?J=∫dm r?=[m/π(R2?-R1?)]∫dθ∫r?dr
θ的积分区间 0--->2π, ?r积分区间 R1--->R2代入积分上下限 积分可得 ：J =[2m/(R2?-R1?)][(R2^4-R1^4)/4]=m(R2?+R1?)/2
取如图面积元dS
dS=rdrdθ
dm=mdS/π(R2?-R1?)=[m/π(R2?-R1?)]rdrdθ
则 ?J=∫dm r?=[m/π(R2?-R1?)]∫dθ∫r?dr
θ的积分区间 0--->2π, ?r积分区间 R1--->R2代入积分上下限 积分可得 ：J =[2m/(R2?-R1?)][(R2^4-R1^4)/4]=m(R2?+R1?)/2