Question

Why can speed of a particle not be negative?

Answer 1

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speed depends upon the distance and distance never be in the negative so the speed also can't be negative.
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Answer 2

Speed is a scalar quantity , it can never be negative as the distance is never negative , distance is the the total length of the part covered by a particle and it has no direction. while displacement is the shortest distance covered by a body it can be negative if the body is going in the opposite direction and the velocity can also be negative as they have particular directions so they are vector quantity ,