Final answer:
When the velocity function is linear, the average velocity over a time interval is the same as the instantaneous velocity at any point within that interval. For non-linear functions, however, the instantaneous velocity at a specific point differs from the average velocity over an interval.
Step-by-step explanation:
The difference between the average velocity over an interval and the instantaneous velocity at a specific point within that interval generally exists because the velocity of an object can change over the period being considered. However, if the velocity function is linear, as noted in the information provided, the average velocity over the interval will be the same as the instantaneous velocity at any given point within that interval. In the case of a linear velocity function, this means that the average velocity between times t=1 and t=2 is the same as the instantaneous velocity at t=2. However, when dealing with non-linear velocity functions, the instantaneous velocity, which is the slope of the tangent line to the position-time graph at a specific instant, can be different from the average velocity, which is the slope of the secant line between two points on that graph.