Final answer:
When the distance travelled by an object is directly proportional to the time, it is said to travel with constant speed, which implies that it has a constant velocity and is moving at a uniform rate without acceleration.
Step-by-step explanation:
When the distance travelled by an object is directly proportional to the time, it indicates a constant speed. This condition means that the object is in linear motion with constant velocity, moving at a uniform rate without change in speed or direction. A practical example of this would be a car driving on a straight road at a steady speed of 60 km/h.
Moreover, in such a scenario, the acceleration is zero because the speed is not changing over time. If we were to look at a velocity vs. time graph for this motion, we would observe a straight horizontal line indicating the constancy of the velocity. This concept is also related to Newton's first law of motion, which states that an object will remain at rest or in uniform motion with constant velocity if no net force acts upon it.
Additionally, uniform circular motion involves an object traveling at a constant speed, but since the direction is changing, the velocity is not constant, and thus it is different from the motion described in the question. It's important to distinguish between uniform linear motion and uniform circular motion in this context.