starting wave at a stop light
Many students confuse a trajecory in space with a path in a
{time, position} plane.
A trajectory in space is series of images perceived by the eye.
A moving point in a {time, position} plane is an image for the mind's eye.
Consider a line of cars waiting for a traffic light. The light turns green
and the cars begin to move.
What moves forward? How fast? What moves backward? How fast?