Here's why:
* Constant Velocity: If an object is moving at a constant velocity (meaning its speed and direction are not changing), its velocity is *not* proportional to elapsed time. The object will cover equal distances in equal intervals of time, regardless of how long it has been moving.
* Acceleration: If an object is accelerating, its velocity is changing. The relationship between velocity and time depends on the type of acceleration:
* Constant Acceleration: In this case, velocity *is* proportional to elapsed time. For example, if an object is accelerating at a constant rate of 2 m/s², its velocity will increase by 2 m/s every second.
* Non-constant Acceleration: If the acceleration is not constant, the relationship between velocity and time becomes more complex. It might be a curve, or a more complicated function.
In summary:
The relationship between velocity and elapsed time depends on the object's motion.
* Constant velocity: Velocity is NOT proportional to time.
* Constant acceleration: Velocity IS proportional to time.
* Non-constant acceleration: Velocity has a more complex relationship with time.
