受力分析和計算(摩擦力和推力)

Let Fb, Fc and Fd be the frictional forces acting between B and C, C and D , D and ground respectively.

Since block A is at rest relative to B, and there is no external force acting, friction betwwen A and B is zero.

Consider block B,
Fb = 4 N

Consider block C
Fb + Fc = 7
i.e. 4 + Fc = 7
Fc = 3 N

Consider block D
Fc + Fd = 12
i.e. 3 + Fd = 12
Fd = 9 N