# Improved Relativistic Mass and Momentum Problem

**Classical Mechanics**Level 5

A particle (A) of rest mass 2\(^{-10}\) kg has a speed of 99.9% the speed of light. Another particle (B) of unknown mass has a speed of only 90% the speed of light. Particles A and B collide in a (linear) perfect elastic collision (the total kinetic energy is conserved). Furthermore, both particles retain the same speed as before the collision took place, but travel in the opposite direction.

Assuming all energy is conserved between the two particles what is the rest mass of Particle B in grams (to 2.d.p)?