Unit 1 described motion. This unit explains it: forces are interactions, the net force sets the acceleration, and every push comes with an equal push back. You will draw free-body diagrams, apply Newton's three laws to systems, and work with gravity, friction, springs and circular motion.
A system's behavior comes from the interactions of its parts, and when the internal details don't matter, the whole system can be treated as one object located at its center of mass. For symmetric objects the center of mass lies on the lines of symmetry.
A force is an interaction between two objects; nothing can exert a net force on itself. Contact forces come from electric forces between atoms. A free-body diagram shows every force on one object or system as an arrow from a dot.
When A pushes on B, B pushes back on A with a force equal in size and opposite in direction. The two forces act on different objects, so they never cancel each other. Internal forces don't move a system's center of mass. Ideal strings have the same tension everywhere.
The net force is the vector sum of all forces. If it is zero (translational equilibrium), velocity stays constant, whether that is at rest or moving steadily. Forces can balance in one direction and not another.
The acceleration of a system's center of mass is the net external force divided by the system's mass, in the direction of the net force. Only external forces change the system's velocity.
Gravity attracts any two masses with a force proportional to each mass and inversely proportional to the square of the distance between centers. Near Earth's surface, g = 10 N/kg and weight is mg. Apparent weight is the normal force, which differs from mg when you accelerate.
Kinetic friction acts on sliding surfaces with size μk·Fn, opposite the relative motion, and doesn't depend on contact area. Static friction takes whatever value prevents slipping, up to a maximum μs·Fn. μs is usually larger than μk.
An ideal spring exerts a force proportional to how far it is stretched or compressed from its relaxed length (Hooke's law), always directed back toward equilibrium.
Moving in a circle requires centripetal acceleration v²/r toward the center, supplied by real forces: gravity, tension, friction, the normal force, or components of them. Tangential acceleration changes speed. Period and frequency describe uniform circular motion, and Kepler's third law relates an orbit's period to its radius.