Types of Motion: BODY

Basic Thermodynamics ~ J. Pohl ©

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The idea, "motion in space," has many aspects. To list a few: the state of rest (no motion relative to Earth), uniform motion, unaltered motion, altered motion, accelerated motion, forced motion, projectile motion, natural motion, free fall and terminal free fall. All of these ideas were in use in the 1700's. Below these terms are contrasted. The system consideration is a mass modeled as a BODY that moves (or does not move) in the surrounding atmosphere. The coordinate space is Cartesian with X and Y axes parallel to Earth (at the origin) and the Z axis directed vertically upward.

Rest Rest is a relative idea. Newton surely understood that objects on Earth, not moving relative to it, were moving nonetheless at great speeds about Earth. This beginning idea, rest, is evidence that Newton's Laws are a limited, Earth-bound perspective.

Uniform Motion These words imply "motion," which would mean not at rest or "moving" relative to earth with that motion being uniform. This means constant velocity being constant speed in a constant direction. The calculus statement, "constant velocity," (of a BODY) is d(mV(t))/dt = 0. There is a special case of "constant velocity." This is that the velocity equals zero at the moment of initial observation and remains zero thereafter. Thus "at rest" is uniform motion with zero velocity. With uniform motion the sum of forces acting on the body is zero.

Unaltered Motion is the same as uniform motion by with emphasis on the sum of forces acting rather than the velocity occurring.

Altered, Accelerated, or Forced Motion Altered and forced motion depict an unbalanced force sum. Accelerated motion is a consequence of being altered or forced.

Uniform and Natural Motion When Galileo and his contemporaries studied terrestrial motion of objects they took did so by components. The horizontal motion of a BODY (at low speeds and for short distances) appeared "uniform." Thus the idea "uniform." Motion was different in the vertical. Earth's gravity was involved; this component of motion was called "netural motion," or motion in accord with nature.

Projectile Motion In these modern times the "BODY" is likely to be the projectile of a cannon involving high speeds and long distances.

Free Fall and Terminal Velocity These motions are generally vertical from a height toward Earth. Free-Fall implies zero effect of surrounding medium (air or water). However, there is drag of the media upon the BODY which increases with speed until a constant, terminal speed (Earthward) is attained.