You are at the controls of a particle accelerator, sending a beam of 3. 90×107 m/s protons (mass m) 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. 60×107 m/s. assume that the initial speed of the target nucleus is negligible and the collision is elastic