I started reading about relativity before doing my first calculus problem, and knew relativity concepts before learning basic physics I concepts. It actually helped me simplify a few problems. I knew that two cars colliding at different speeds was just another description of one car colliding into a stationary one at the net speed. However, as I learn more about early physics, it also requires me to go back to my understanding of relativity. Learning about kinetic energy, for example. I can only assumr that the amount of kinetic energy an object possesses is relative. But, energy of a closed system is constant. How do I reconsile these two facts? Is the energy still there, but "where it is" is relative? If I observe a friend moving at 50 meters per second, he and I both have relative velocities, and either of us could possess that kinetic energy? But that leads me to my next problem. If my friend is 10kg heavier than me, we are both traveling at 50 m/s relative to one another, but relative to me he is traveling with a greater kinetic energy than I am relative to him. How does that follow a constant amount of energy?