89.1 Momentum and Newtons second law of motion (e) Internal and external forces The forces from other particles within the system of particles is called internal force. The forces from sources outside the system of particles is called external force. The total force of the system: F1+F2+…+F2+… Flin t Fle+ F2in t Fret =F1++F2+…+F+… 89.1 Momentum and Newtons second law of motion According Newton’ s third law Fu F2r F23 ∑Fm=0 F3n F13 se f= F+ F 1 in dt =F.+F= dt PN N F、+F N dt7 The forces from other particles within the system of particles is called internal force. The forces from sources outside the system of particles is called external force. The total force of the system: (e) Internal and external forces L r L r r L r r r r r L r L r r r = + + + + + = + + + + = + + + + + 1ex 2ex i ex 1in 1ex 2in 2ex total 1 2 F F F F F F F F F F Fi FN §9.1 Momentum and Newton’s second law of motion §9.1 Momentum and Newton’s second law of motion m1 m2 m3 F12 r F21 r F13 r F31 r F32 r F23 r F1ex r F3ex r F2ex r According to Newton’s third law in = ∑ in ≡ 0 i F Fi r r t p F F F t p F F F t p F F F N N N N d d d d d d ex in 2 2 2 ex 2 in 1 1 1ex 1in r r r r LLL r r r r r r r r = + = = + = because = + =