In one dimension, there is only one velocity component and the speed is just the absolute value of that component.

Velocity = - 300 meters/sec,
Speed = 300 meters/sec.

Two Dimensions: An airplane is losing altitude at a rate of 300 meters per second while moving horizontally at a rate of 400 meters per second. What is it's speed?
In one second the plane travels:
400 meters horizontally and 300 meters down. The distance it travels is found (by using geometry) to be 500 meters, so its speed is 500 m/s.

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