取如圖面積元dS
dS=rdrdθ
dm=mdS/π(R2?-R1?)=[m/π(R2?-R1?)]rdrdθ
則 ?J=∫dm r?=[m/π(R2?-R1?)]∫dθ∫r?dr
θ的積分區間 0--->2π, ?r積分區間 R1--->R2代入積分上下限 積分可得 ：J =[2m/(R2?-R1?)][(R2^4-R1^4)/4]=m(R2?+R1?)/2
取如圖面積元dS
dS=rdrdθ
dm=mdS/π(R2?-R1?)=[m/π(R2?-R1?)]rdrdθ
則 ?J=∫dm r?=[m/π(R2?-R1?)]∫dθ∫r?dr
θ的積分區間 0--->2π, ?r積分區間 R1--->R2代入積分上下限 積分可得 ：J =[2m/(R2?-R1?)][(R2^4-R1^4)/4]=m(R2?+R1?)/2