In this section the laws of motion applicable to a continuous medium analogous to Newton s laws of motion in particle mechanics will be formulated. Newton s laws of motion are first reviewed below.
Newton s first law of motion: In an inertial frame of reference, a free particle continues in its state of rest or of uniform motion.
Newton s second law of motion: In an inertial frame, the rate of change of linear momentum of a particle equals the resultant force acting on it. That is,
where it has been assumed that the mass m of a particle does not change. Here 
denotes a force acting on the particle. The linear momentum of a particle equals its mass multiplied by its velocity.
Newton s third law of motion: To every action there is an equal and opposite reaction.
Newton s first law of motion defines an inertial frame: an inertial frame is one in which Newton s first law of motion holds. Usually, it is taken as a frame attached to the sun. However, in most engineering problems, one can take coordinate axes fixed to Earth as an inertial frame without introducing an appreciable error. Hereafter, an inertial frame will be taken as the frame of reference.
To write the laws of motion for a continuum, we note that the linear momentum of the material enclosed in an infinitesimal volume dv is ( ? dv) v
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