1. The problem statement, all variables and given/known data An observer on Earth sees two spaceships moving in opposite directions and finally they collide. At t=0 the observer on Earth says that the spaceship 1 which moves to the right with Ua=0.8c is at the point A and the spaceship 2 which moves to the left with Ub=0.6c is at the point B. The distance AB=L=4,2.10^8 m. When do the two spaceships collide to the earth frame of reference? What is the velocity of the spaceship 2 to the frame reference of the spaceship 1? What is the velocity of the spaceship 1 to the frame reference of the spaceship 2? When does the collision happen to the frame reference of the spaceship 1 and to the frame reference of the spaceship 2? 2. Relevant equations 3. The attempt at a solution Let D be the point of the collision and AD=x, so DB=L-x The velocity is constant. spaceship 1: AD=x=0,8c.t spaceship 1: DB=L-x=0,6c.t By addition L=AD+DB L=(0,8c+0,6c).t t=4,2.10^8/(1,4.3.10^8) t=1sec Is it ok so far? A hint to go on, please?