任中仁
幼苗
共回答了20个问题采纳率:90% 举报
设棒上一微元,长dx,-½L ≤ x ≤ ½L (L为棒长)
微元所在处与中垂线上距棒a处的P点连线,
连线与中垂线的夹角为θ.
微元的质量:dm = ρdx
微元与P的距离的平方:x²+a²
根据万有引力公式,得微元与P点处的质点的引力为
dF = Gmρdx/(x²+a²) (G:万有引力常数)
合力 F = ∫dF×cosθ (x:-½L→½L)
其中 cosθ = a/√(x²+a²)
F = ∫[Gmρdx/(x²+a²)]×cosθ (x:-½L→½L)
= Gmρ∫dxcosθ/(x²+a²) (x:-½L→½L)
= Gmρa∫dx/(x²+a²)^(3/2) (x:-½L→½L)
= 2Gmρa∫dx/(x²+a²)^(3/2) (x:0→½L)
设 x = atan u,x :0→½L; u :0→arctan(L/2a)
(x²+a²)^(3/2) = (a²tan²u+a²)^(3/2) = a³/cos³u
dx = (a/cos²u)du
F = 2Gmρa∫(a/cos²u)du/[a³/cos³u] (u:0→arctanL/2a)
= (2Gmρ/a)∫cosudu (u:0→arctanL/2a)
= (2Gmρ/a)sinu (u:0→arctanL/2a)
= (2Gmρ/a)×L/√(L²+4a²)
= (2GmρL/[a√(L²+4a²)]
1年前
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