F=F+F2+F+…=F1=md net That is a=(F1+F2+F3+…)=a1 u十 which is called as the principle(原理)of superposition of forces or principle of independence of forces(叠加,独立)
(F F F ) a a .... m a + + + = + + = Fnet F F F Fi ma = + + + = = That is m F a i i = which is called as the principle （ 原 理 ） of superposition of forces or principle of independence of forces（叠加, 独立）. F F F a
(3)The relation F=ma holds(5X3) instantaneously F No force. no acceleration!
⑶ The relation F ma holds（成立） instantaneously. = F a No force, no acceleration!
(4)The components( in two dimensions) of(2-1)are ∑F.=m=mx”↑p dt ∑ F=ma y J dt
x y o P r ⑷ The components ( in two dimensions) of (2-1) are d d 2 2 t x Fx = ma x = m d d 2 2 t y Fy = ma y = m F a