Which fundamental force binds protons together in the atomic nucleus?

• A. Electromagnetic force
• B. Weak nuclear force
• C. Gravity
• D. Strong nuclear force

Answer: (D)

Inside the nucleus, protons would repel each other because they carry positive charge, but a force that acts over extremely short distances binds them together. This is the strong nuclear force, the most powerful of the fundamental interactions, and it operates at about a femtometer (10^-15 meters). It binds protons and neutrons into nuclei by acting between nucleons, overcoming the electromagnetic repulsion that would otherwise tear the nucleus apart. Gravity is negligible at atomic scales, and the weak nuclear force is responsible for certain decay processes rather than holding nuclei together. So the force actually doing the binding is the strong nuclear force.