Question

"Instantaneous speed is always equal to the magnitude of instantaneous velocity". WHY???

Answer 1

see,
#speed is a scalar quantity.
and velocity is a vector quantity.
and being a vector,it has direction.
#so first of all the reason why we take magnitude of velocity is that it can be positive or negative, but speed is only positive it can't be negative as it isnt vector. not both can be equated.
# now comes instantaneous means for very small interval nearly equal to 0. for very small fraction of second.
So, there we can say that time is equal and also distance is equal to displacement because here very small distance we.are.taking.(see the image).

we.know that speed is distance/time and velocity is dispalcement /time.
and here as i have mentioned both are equal.
hope this answer helps you.

please ask if you have any doubt.

Answer 2

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Instantaneous velocity. The rate of change of displacement of an object in a particular direction is its velocity.The magnitude of instantaneous velocity equals the instantaneous speed. This happens because, for an infinitesimally small time interval, the motion of a particle can be approximated to be uniform.