Lumped Electromechanical Elements systems this is a mass and for rotational systems a moment of inertia. After we have reviewed the plane motion of a point mass, we shall generalize to translation of a rigid body of finite size and to rotation of a rigid body about a fixed axis. The motion of a point that has associated with it a constant amount of mass M is described by Newton's second law*: f = M dv, (2.2.9) dt where f is the force vector acting on the mass point and v is the absolute vector velocity of the point. It should be clearly recognized that v must be measured in relation to a fixed or nonaccelerating point or frame of reference. Such a reference system is called an inertialreference. In motion occurring on the surface of the earth the earth is often considered to be approximately an inertial reference and v is measured in relation to the earth. When dealing with the motion of long-range missiles, orbital vehicles, and spacecraft, the earth cannot be considered to be a
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