The momentum of an electron traveling at 0.8c cannot be accurately calculated using the classical equation p = mv, as this is only valid at low speeds. Instead, the relativistic momentum formula should be used: p = m0v/√(1-v²/c²), where m0 is the rest mass of the electron. At 0.8c, the relativistic effects become significant, necessitating this adjustment for accurate calculations. The discussion highlights the importance of using the correct equations when dealing with high-speed particles. Understanding these concepts is crucial for solving related physics problems effectively.