Heisenberg's uncertainity principle states that we can't mutually determine both the trajectory and momentum of a subatomic particle. BUT when dealing with moving charges and magnetism we do problems with the charge in electromagnetic fields. the path is sometimes cicular, helical, cycloid etc.
Consider an electron fired with a velocity $v$. We know its path in a perpendicular magnetic field is a circle with radius $\frac{mv}{qB}$.
The uncertainty principle is true in a quantum mechanical framework. The problems you mention are instead in a classical framework and because of this Heisenberg's principle doesn't apply.
The fundamental reason why Heisenberg's principle is true in quantum mechanics is that in the quantum theory observables such as position and momentum are, mathematically, operators and not scalar quantities. As operators, they don't, in general, commute, which means $AB\neq BA$ in general for two operators $A$ and $B$. This is however not true in classical physics, where the product is always commutative.
