Final answer:
A horizontal line on a velocity-time graph indicates that the object is moving at a constant velocity with zero acceleration. A constant positive or negative gradient on this graph represents a constant positive or negative acceleration, respectively.
Step-by-step explanation:
From the provided information, it's clear that a velocity-time graph can help us determine an object's motion characteristics, such as velocity, acceleration, and displacement. When analyzing such a graph, a horizontal line indicates that the object is moving at a constant velocity, with zero acceleration since there are no changes in velocity. Thus, if the velocity is constant and not changing, this implies a horizontal line on a velocity-time graph - no slope means no acceleration. When the graph shows a positive gradient that is constant over a period, this indicates a positive acceleration, and likewise, a negative gradient that is constant implies negative acceleration. Consequently, if a scenario describes a period where the object is not changing its speed, it has zero acceleration, as reflected by a horizontal line on the graph.
In the case of increasing or decreasing velocities, if the change in the velocity is consistent, then this would be depicted as a straight line with either a positive or negative slope. For an acceleration-time graph, sections with zero acceleration would be represented as horizontal lines at the zero mark, while constant accelerations would be indicated by horizontal lines above or below this mark, depending on whether the acceleration is positive or negative.