1. How close would two stationary electrons have to be positioned so that their total mass is twice what it is when the electrons are very far apart? 2. p = (mv) / (squareroot(1-(v2/c2)) E = (mc2) / (squareroot(1-(v2/c2)) E = mc2 m = (rest mass) / (squareroot(1-(v2/c2)) L = (proper length)*(squareroot(1-(v2/c2)) 3. m = 2(rest mass) m = (rest mass) / (squareroot(1-(v2/c2)) v = 6.75 * 1016 I solved up to the velocity but I don't know how to calculate the distance. 1. A spacecraft approaching the earth launches an exploration vehicle. After the launch, an observer on earth sees the spacecraft approaching at a speed of 0.50c and the exploration vehicle approaching at a speed of 0.70c. What is the speed of the exploration vehicle relative to the spaceship? 2. u = (u' + v) / (1 + (u;v/ c2)) u' = (u - v) / ( 1- (uv/c2)) 3. I attempted this problem by setting u' equal to each other (each u either represents the exploration vehicle or the spaceship). But, I got stuck. I didn't know which v was for which vehicle/spaceship.