Randall D. Knight — Interactions and Potential Energy: Problem 75

Protons and neutrons (together called nucleons) are held together in the nucleus of an atom by a force called the strong force. At very small separations, the strong force between two nucleons is larger than the repulsive electrical force between two protons—hence its name. But the strong force quickly weakens as the distance between the protons increases. A well-established model for the potential energy of two nucleons interacting via the strong force is \[ U = U_0 [1 - e^{-x/x_0}] \] where \( x \) is the distance between the centers of the two nucleons, \( x_0 \) is a constant having the value \( x_0 = 2.0 \times 10^{-15} \text{ m} \), and \( U_0 = 6.0 \times 10^{-11} \text{ J} \). Quantum effects are essential for a proper understanding of nucleons, but let us innocently consider two neutrons as if they were small, hard, electrically neutral spheres of mass \( 1.67 \times 10^{-27} \text{ kg} \) and diameter \( 1.0 \times 10^{-15} \text{ m} \). Suppose you hold two neutrons \( 5.0 \times 10^{-15} \text{ m} \) apart, measured between their centers, then release them. What is the speed of each neutron as they crash together? Keep in mind that both neutrons are moving.