Two balls with same mass m1 and m2 are moving in a straight line by velocity v1 and v2 with respect to the ground. They then collide elastically with each other. That is, the total momentum and the total kinetic energy of the two balls are conserved in the collision.
1. Please discover the final velocities of the two balls.
2. If you are driving a car with a velocity u in the direction of the moving of the balls, please prove that under Galilean relativity in your observation the total momentum and the total kinetic energy of the two balls are still conserved, regardless of your velocity u.