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, p. 886)


Speed is “the rate of movement or motion.”  (Id., 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.”  (Id., 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.[1]


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.”  (Id., p. 396)


Relative displacement is the distance of displacement of one body or thing relative to another body or thing.

[1] In Special Relativity inertial motion may result from a force.