What is the difference between the following concepts: motion, speed, velocity, acceleration, inertial motion, relative motion, relative velocity, displacement, and relative displacement?
Motion is “the passage of a body from one place to another” (Webster’s Dictionary).
Speed is “the rate of movement or motion” (Ibid, p. 1288). Specifically, ‘speed’ is the distance that a body moves from one position to another position, divided by the time elapsed: S = d/t.
Velocity is “the rate of change of position [of a body] in relation to time…in a particular direction” (Ibid, p. 1479).
Uniform rectilinear velocity is where the speed of a body is constant in the same rectilinear direction.
Acceleration occurs when the speed or the direction of a body’s velocity is changed.
Inertial motion (in classical physics) is where the uniform rectilinear velocity of a body does not result from an apparent force. In Special Relativity inertial motion may result from a force.
Relative motion is the motion of one body relative to the motion of another body.
Relative velocity is the rate of change of position of one body (or thing) in a particular direction as compared to the rate of change of position of another body (or thing) in its particular direction.
Displacement is “the difference [in distance] between a later position of a thing and its original position” (Ibid, p. 396).
Relative displacement is the distance of displacement of one body or thing relative to another body or thing.