Gravity vs. electricity *

(a) In the domain of elementary particles, a natural unit of mass is the mass of a nucleon, that is, a proton or a neutron, the basic massive building blocks of ordinary matter. Given the nucleon mass as 1.67 · 10-27 kg and the gravitational constant G as 6.67 · 10-11 m3/(kg s2), compare the gravitational attrac- tion of two protons with their electrostatic repulsion. This shows why we call gravitation a very weak force.

(b) The distance between the two protons in the helium nucleus could be at one instant as much as 10-15 m. How large is the force of electrical repulsion between two protons at that dis- tance? Express it in newtons, and in pounds. Even stronger is the nuclear force that acts between any pair of hadrons (includ- ing neutrons and protons) when they are that close together.