Simulation: How would you simulate a law of conservation of energy?

I am new in computer science, but I have studied physics for a while. I'm just curious, if one wanted to do a simulation of the law of conservation of energy, on a large scale, how would it be done? Most simulations take shortcuts for objects off the screen, or they can even skip them completely during iterations. Objects can freeze instead of the screen, or group together if they do not interact. Can these tricks be applied by simulating the conservation of energy, without violating the law? Thank you.