5.8 Relativistic Momentum

What is the momentum of an electron traveling at a speed 0.985c? The rest mass of the electron is $9.11 \times 10^{−31} \text{kg} .$

Is it possible for an external force to be acting on a system and relativistic momentum to be conserved? Explain.

Find the momentum of a helium nucleus having a mass of $6.68 \times 10^{−27} \text{kg}$ that is moving at 0.200c.

(a) Find the momentum of a $1.00 \times 10^{9} -\text{kg}$ asteroid heading towards Earth at 30.0 km/s. (b) Find the ratio of this momentum to the classical momentum. (Hint: Use the approximation that $\gamma = 1 + \left(1 / 2\right) v^{2} / c^{2}$ at low velocities.)

What is the velocity of an electron that has a momentum of $3.04 \times 10^{−21} \text{kg} \cdot \text{m}/\text{s}$ ? Note that you must calculate the velocity to at least four digits to see the difference from c.