Systems, forces and free-body diagrams

Learning objectives

Define a system and its center of mass, and explain when a collection of objects can be modeled as a single point particle and when its internal structure matters

Identify the forces acting on an object, distinguishing contact forces such as normal force, friction, tension and spring force from the long-range gravitational force

Construct a free-body diagram in which each force is a labeled vector drawn from a single point with length matching relative magnitude, and resolve the forces onto chosen axes

Apply Newton's third law to identify a force pair, explaining that the two forces act on different objects, are of the same type and are equal in magnitude but opposite in direction

State Newton's first law, define translational equilibrium, and use it to find unknown forces on an object at rest or moving at constant velocity

Apply Hooke's law to an ideal spring, determine the spring constant from the slope of a force–extension graph, and explain the meaning of the negative sign in F = −kx