Introduction

Every unit until now treated bulk bodies as if they were single point objects, ignoring size and shape entirely. Real bodies, however, are made up of an enormous number of individual particles, and when such a body moves, its motion is really the combined motion of this whole collection of particles. To describe this combined motion with a single, simple picture, we introduce the center of mass — one special point that represents the entire body's translational behaviour, no matter how the individual particles within it may be moving relative to one another.

The forces that act on a bulk body fall into two categories. Internal forces are the forces the particles of the body exert on each other (e.g. the cohesive forces holding a solid together); external forces are forces coming from outside the body altogether (gravity, an applied push, friction from another surface). This distinction matters throughout the unit — internal forces can never change a body's center-of-mass motion or its total momentum, only external forces can.

A rigid body is, by definition, one that keeps a completely fixed shape and size under any external force — the distance between any two particles of the body never changes, however hard it is pushed, pulled or twisted. No real object is perfectly rigid (every material deforms at least a little under stress, a topic taken up separately under elasticity), but treating a body as rigid is an excellent approximation for a huge range of everyday and engineering problems, and it is the approximation used throughout this unit to study center of mass, torque and angular momentum, equilibrium, moment of inertia, rotational dynamics, and rolling motion.