18 Questioning the Universe: Concepts in Physics
Graph 1: This is the same as graph 2 for constant speed. Since the speed is
constant, the acceleration is zero. Even though zero is a constant, zero
acceleration is a special case and we want to distinguish it from non-
zero acceleration. Also, the line is drawn above the origin, which shows
the object has a positive speed (e.g., it is moving toward the right). The
object could have been moving to the left (negative speed), so the line
would have been drawn below the origin. In either case, the acceleration
is zero.
Graph 2: Here the acceleration is the dependent variable and has a constant
positive value. If the line were drawn below the origin, the acceleration
would be negative. We will se ...