You are at the controls of a particle accelerator, sending a beam of 3.60×10^7 m/s protons (mass ) at a gas target of an unknown element. Your detector tells you that some protons bounce straight back after a collision with one of the nuclei of the unknown element. All such protons rebound with a speed of 3.30×10^7 m/s. Assume that the initial speed of the target nucleus is negligible and the collision is elastic. Find the mass of one nucleus of the unknown element. Express your answer in terms of the proton mass m.