3节数量积与向量积.pdf

3 r M F 1 r r M s F 2 r r r r θ F s W | F || s | cosθ ( ) , . r r r r a b a ⋅b r r r r r r a ⋅b | a || b | cosθ θ a b ( ) r r r b r r θ a ⋅b | a || b | cosθ r a r r r r Q| b | cos θ Pr j b , | a | cos θ Pr j a, a b r r r r r r ∴a ⋅b | b | Pr j b a | a | Pr j a b . . r r r 2 (1) a ⋅a | a | . r r r r r 2 Qθ 0, ∴a ⋅a