Strong and Weak Force

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The strong and weak forces are the forces that govern the subatomic behavior of atoms.

This is a short introduction to the nuclear strong and weak forces, the other two of the four fundamental forces. These two forces will not be covered in depth by this class, and as such this is just a general overview.

Main Idea

The strong nuclear force governs interactions on a scale of [math]\displaystyle{ 1 fm = 10^{-15} m }[/math]. The strong force is [math]\displaystyle{ 10^{38} }[/math] times stronger than the force of gravity, and 137 times stronger than the electromagnetic force. It is the strongest of the four fundamental forces (gravity, electromagnetic force, strong nuclear force, and weak nuclear force). The strong force is what binds neutrons and protons together to create nuclei. Because it is stronger than the electromagnetic force, it allows particles with the same charge sign to be compressed together despite their revulsion due to the electromagnetic force. On an even smaller scale, the strong nuclear force is what holds quarks together. When quarks are held together, they create particles such as protons and neutrons. Quarks are a class of subatomic molecules that, when combined, create hadrons, the most stable of which are protons and neutrons. Electrons are NOT hadrons.

The weak nuclear force, on the other hand, governs interactions on a scale of [math]\displaystyle{ 10^{-17} m }[/math]. The weak force in atoms causes radioactive decay in the atom, and is the force that allows neutrons to decay into protons, etc.


Prior to the 1970s, it was unclear how the nucleus managed to hold itself together. Classical electromagnetism implies that nuclei shouldn't stay bound together. A strong attractive force, greater in magnitude than the repulsion due to electromagnetic force was needed to explain what held the extremely positively charged nucleus together. It was in the 1970s that the nuclear strong force was first proposed.

With regards to the weak force, Enrico Fermi was fundamental in proposing the idea for it. In 1933, he suggested a mechanism to explain beta decay (which occurs when a neutron decays into a proton, an electron, and an antineutrino). The mechanisms behind this force were not directly confirmed until 50 years later.