Nuclear forces are so strong that they keep protons and neutrons in a spherical region a few femtometers in radius (1 fm = 10⁻¹⁵ m). To get a crude idea of the energy scales involved in nuclear physics, model the nucleus as a cube 5 fm on a side. What is the ground state energy of a proton (mass mp ≅ 1.7 × 10⁻²⁷ kg) in this box? This is the energy scale of nuclear interactions.