Simply put HUP involves position and momentum, further more consider a mass of 1kg. as momentum is mass X velocity = 1X velocity = velocity for calculation purposes. now for a stationary observer the uncertainty in velocity of the observed mass is some value, if another observer is travelling at the same speed as the mass the relative velocity is zero or at least smaller than what the stationary observer would see the mass travelling at. we now have two values for the uncertainty in the velocity of the mass. they both can't be right, HUP is subject to classical Newtonian relativity?