Question

Why isn't average speed defined as the magnitude of average velocity?

Answer 1

Even if the object is slowing down, and the magnitude of the velocity is decreasing, its direction would be still the same as the direction in which the object is moving. The magnitude of average velocity is always less than or equal to the average speed because displacement is always smaller than or equal to distance.

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Answer 2

because it is not necessary that magnitude of velocity and speed is same .velocity depends upon displacement whereas speed depends upon distance .for an example let's say I start walking from my house and covers 30 km and return to my home in 15 hours then the speed will be 2 km/ hour but velocity is zero since displacement is zero .i
I hope this will help you