(2) Motion. Newton's Laws of Motion
75. Motion a Change of Position.—Motion is defined as a continuous change in the position of a body. The position of a body is usually described as its distance and direction from some fixed point. Thus a man on a boat may be at rest with respect to the boat and moving with respect to the earth. Or, if he walks toward the stern as fast as the boat moves forward, he may keep directly over a rock on the bottom of the lake and hence not be moving with reference to the rock and yet be in motion with respect to the boat. Motion and rest, therefore, are relative terms. The earth itself is in motion in turning on its axis, in moving along its orbit, and in following the sun in its motion through space. Motions are classified in several ways:
(A) Modes of Motion
1. Translation.—A body is said to have motion of translation when every line in it keeps the same direction.
2. Rotation.—A body has motion of rotation when it turns upon a fixed axis within the body, as a wheel upon its axle or the earth upon its axis.
3. Vibration or Oscillation.—A body is said to have vibratory or oscillatory motion when it returns to the same point at regular intervals by reversals of motion along a given path, e.g., a pendulum.
(B) Direction of Motion
1. Rectilinear.—A body has rectilinear motion when its path is a straight line. Absolute rectilinear motion does not exist, although the motion of a train on a straight stretch of track is nearly rectilinear.
2. Curvilinear.—A body has curvilinear motion when its path is a curved line, e.g., the path of a thrown ball.