Statics @The invariants of the reduction of a general coplanar force system to RER R a given center The invariants of the reduction of a general force system in space to a given center are R in a plane: R In space: R ,=(RR (4) The way to judge the direction of the force of friction Force of friction is a sort of constraint reaction force Its direction is always opposite to the relative motion of the bodies5 ③The invariants of the reduction of a general coplanar force system to a given center. The invariants of the reduction of a general force system in space to a given center are in a plane: in space: R' // R' ; M M =(R R) ⊥ , ④ The way to judge the direction of the force of friction: Force of Friction is a sort of constraint reaction force. Its direction is always opposite to the relative motion of the bodies.