Final answer:
A speed-time graph for a particle moving at a constant speed is depicted as a straight line parallel to the time axis, indicating uniform motion and zero acceleration. This representation reflects the concept that the particle's speed does not change over time.
Step-by-step explanation:
When analyzing a particle's movement in physics, a speed-time graph can reveal important information about its motion. A speed-time graph depicting a particle moving at a constant speed is exemplified as a straight line that is parallel to the time axis. This indicates that the speed does not change over time and that the particle is in uniform motion. In contrast, if a particle's speed changes, the graph would show a curve with varying slopes indicating acceleration. The slope of a speed-time graph corresponds to the particle's acceleration. Thus, a straight horizontal line reflects zero acceleration, which is consistent with a constant speed.
Furthermore, the world line of a particle provides a broader perspective in spacetime. A stationary particle has a world line that is a vertical line parallel to the time axis, while the world line for a particle with constant velocity is a straight line inclined to the time axis. When considering a velocity vs. time graph, a horizontal line represents a constant velocity. The area under such a graph can be used to calculate the displacement of the particle over a given period.