A proton (mass 1.67 × 10⁻²⁷ kg, charge +e) is fired directly at a lithium nucleus (mass 1.16 × 10⁻²⁶ kg, charge +3e). If the proton's velocity is 5.24 × 10⁵ m/s when it is far from the nucleus, how far apart will the two particles be when the proton is at rest, just before it turns around? Assume the nucleus is free to recoil.