Step-by-step explanation:
Heisenberg uncertainty principle.
the thing about velocity is that it is about movement :
DISTANCE / time (difference between start and end)
so, to measure velocity you need to observe the object during a period of time (even if it is a tiny period).
what you get is a mean value of the velocity the object was moving with while going from point A to B during the observed time period.
now, at what point between A and B did it have exactly that velocity ? we can't say, due to the nature of velocity.
no matter how tiny we make the distance between A and B and the time interval, it has to be a measurable distance and time interval. they cannot be 0.
otherwise we would have a velocity 0/0. and that is undefined.
the other direction is to identify the position of a moving body. once we have the velocity, we are dealing with a distance and not a point. the velocity of an object at a point is
0/time = 0
so, for a given object we can only determine the exact position at a given time OR the exact velocity it has during a given time interval.
both is simply impossible.