A force-couple system is a combination of two or more forces acting on an object, where the forces do not intersect at a common point and their resultant is a pure moment or torque.
A single force has a point of application and a line of action, while a force-couple system has a line of action but no point of application. Additionally, a single force produces translational motion, while a force-couple system produces rotational motion.
The three forces that can be replaced by a force-couple system are parallel forces, non-parallel forces, and a single force with a moment acting on it.
The resultant moment in a force-couple system is determined by finding the algebraic sum of the individual moments of each force in the system.
Some examples of force-couple systems in everyday life include opening a door, turning a steering wheel, and using a wrench to tighten a bolt. These actions all involve the combination of two or more forces to produce a rotational motion.