Average Speed

Average speed is the total distance an object travels divided by the total time it takes, in meters per second. It is a scalar, it is never negative, and it is not the magnitude of the average velocity unless the motion never turns around.

Worth knowing before you look for it: the AP Physics 1 course framework defines average velocity and names speed as a scalar, but it gives no separate definition or equation for average speed, and the equation sheet has no line for it either. Its symbol list runs vv = velocity or speed, one entry for both.

So it arrives as a lab quantity rather than a framework quantity. The CED's photogate section is where it earns its keep: the speed a pair of photogates measures may be the object's instantaneous speed at one location or the average speed between two different gates, depending on the setup, and the CED says that in some applications the distinction is crucial to obtaining the appropriate value.

average speed=total distancetotal elapsed time\text{average speed} = \frac{\text{total distance}}{\text{total elapsed time}}

Distance on top, not displacement. That is the entire difference from average velocity, and it is why the two disagree on any path that bends. A satellite in a circular orbit completes one revolution with a large average speed and an average velocity of exactly zero, because it finished where it started.

Averaging speeds directly is the trap. Two equal-length legs at 30 m/s and 60 m/s do not average 45 m/s. The slow leg takes twice as long, so it dominates the total time, and the answer is 40 m/s. Divide total distance by total time every time rather than averaging the numbers you were handed.

More on the scalar and vector pair in speed and speed vs velocity.

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