1. The problem statement, all variables and given/known data An electron is confined to a nucleus of radius 4 femtometres. Estimate its minimum energy. 2. Relevant equations [itex]ΔxΔp_x=h/4\pi[/itex] [itex]E^2=p^2c^2 + m^2c^4[/itex] As the electron's rest energy will be much less than it's kinetic energy, [itex]E=pc[/itex] 3. The attempt at a solution So I understand that I'm supposed to use the uncertainty principle to find the average momentum, and then plug it into the energy equation. What I don't understand is that the solutions use Δpx/2 instead of just Δpx. Why is this?