I'm startled due to a little thinking, the thing is. (I'm still a beginner when it comes to mechanics so please don't give me way to advanced answers ) Think of a mass m, that accelerates with acceleration a. The accelerating force upon the mass m results into a force, F. I'm totally down with that but. * The object m is accelerating, which means the speed is constantly increasing. Now let's say an object accelerates with 1m/s^2. Give it enough time and it will reach the speed of light, right? Then take another object with the same mass m, accelerating at 2m/^2. It will also reach the speed of light if you give it enough time. But, the force F, is twice as much (right?). And when they both are at 99.99% speed of light, and imagine they collide (moving opposite directions). What will really happen? They both were at the same speed. But the second object m*2=F... They are moving towards each other and when they collide, they have exactly the same speed (=same amount of kinetic energy?), BUT, the second object had an acceleration that was twice as much as the first object. But will the (2F) do any difference during the impact? * Let's say now I push the first object and give the constant speed 99.99% speed of light. At the same time the opposite direction object two comes towards it. The collide with the same speed once again, but the first object was not accelerating, the second one was. They both had the same speed as I mentioned which means the same amount of kinetic energy? But! The second object had this equation along, m*a=F. Does this result into an impact when the second object as the dominating force? Or are they both equal? Gosh now it's getting tough in the head :shy:... I hope you understand what I'm trying to understand here.