In quantum mechanics, the Heisenberg Uncertainty Principle states that the more precisely the position of a particle is known, the less precisely its _____ can be known.

The principle says that if you pin down a particle’s location very precisely, you cannot know its momentum very precisely, and vice versa. This happens because measuring the position forces the wave function to become very spread out in momentum space. The uncertainty relation is Δx Δp ≥ ħ/2, so making Δx small makes Δp large. For example, if you measure an electron’s position within 10⁻¹⁰ m, its momentum uncertainty will be at least about 10⁻²⁵ kg·m/s. Thus the missing word is “momentum.