Classical Mechanics

1)

in the following I mean Fx and Fy

2)

The following equations are vector quantities:
dP1/dt = F1/2 + F2/1
dP2/dt = F2/1 + F2/3
dP3/dt = F3/1 + F3/2

dP/dt = F1/2 +F1/3 + F2/1 + F2/3 + F3/1 + F3/2 = 0