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Introduction

This chapter deals with a branch of mechanics that is concerned with bodies at rest or in motion acted upon by forces. The bodies in statics and dynamics are treated as rigid bodies; that is, the bodies undergo no deformation. Thus, statics and dynamics form a branch of mechanics called rigid-body mechanics.

Statics deals with bodies that are in equilibrium with applied forces. Such bodies are either at rest or moving at a constant velocity. On the other hand, dynamics deals with bodies that are accelerating. For an accelerating system, the velocity of the system is not constant.

Rigid-body mechanics is based on the Newtons' laws of motion. These laws were postulated for a particle, which has a mass, but no size or shape. Newton's laws may be extended to rigid bodies by considering the rigid body to be made up of a large number of particles whose relative position from each other do not change. Newtons' laws may be stated as follows:

First law. A particle at rest or in motion with constant velocity along a straight line will remain in its present state unless acted upon by an unbalanced force.

Second law. If an unbalanced force acts upon a particle, then the particle will experience an acceleration that has the same direction as the force. The magnitude of the acceleration is proportional to the magnitude of the force.

Third law. For every force acting on a particle, the particle responds with an equal and opposite reactive force.

Rigid-body mechanics is a foundation of mechanical engineering. Because this subject deals with forces applied to a body, a mechanical engineer must be able to obtain a representation of the body with the applied forces. This representation is called a free-body diagram. Furthermore, to predict the response of the body under the applied forces, the engineer must be able to mathematically represent the forces. Such forces may be mathematically represented as vectors.