Question

How to measure the mass of an uncharged particle like a neutron?

Answer 1

Compare alpha particles and neutrons travelling at the same speed. Use an alpha particle (two protons, known mass and a +2 charge plus two neutrons with no charge and an unknown mass). Compare its trajectory to that of a single proton with a known mass and a +1 charge when deflected in an electric field. In practice, this is done by mass spectroscopy.