Question

Let $F_{pp}$ and $F_{nn}$ denote the net magnitudes of forces between a proton and a proton, a proton and a neutron, and between two neutrons, when they are $1fm$ apart. Which is the correct relation?

• A. $F_{pp}=F_{pn}=F_{nn}$
• B. $F_{pp}<F_{pn}=F_{nn}$
• C. $F_{pp}<F_{pn}<F_{nn}$
• D. $F_{pp}=F_{pn}<F_{nn}$

Answer 1

The correct option is B $F_{pp}<F_{pn}=F_{nn}$

The strong nuclear force is independent of electric charge. Which is to say, if two protons, or a proton and a neutron, or two neutrons are brought very close to each other $(\sim 1fm)$ each pair will feel the same strong nuclear force.


But in the first case, where two protons are kept very close to each other, the Coulomb repulsion will be very high, and will result in a decreased net force between the particles. In the two other cases, Coulomb repulsion will be absent altogether, since we don't have two charges. Therefore, denoting the net force between the particles by $F$ we can say $F_{pp}<F_{pn}=F_{nn}$