The Passing Lane
Observe the two cars below. The blue car starts ahead of" the red car. (The red car actually starts off the screen.) Since the red car is moving faster, it eventually catches up with and passes the blue car. Observe the velocity-time graphs for these two cars. Each car's motion is represented by a horizontal line, indicating a constant velocity. Observe that even though the cars pass each other, the lines on the velocity-time graphs do not intersect. Since the cars never have the same velocity, the lines on the velocity-time graph never cross. The lines would intersect for a position vs. time graph; the fact that the red car passes the blue car means that there is an instant in which they occupy the same position. The two cars have the same position at seven seconds; yet they never have the same velocity at any instant in time.
For more information on physical descriptions of motion, visit The Physics Classroom Tutorial. Detailed information is available there on the following topics:
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