MEMO 3.3

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.