title: How does a physics engine actually simulate physics?
body: <p>This question may be a stupid question, but I'm really curious.</p>
  
  <p>After playing games like HL2, GMod, or Angry Bird, and using physics libraries like Box2D, I began to wonder <strong>"how a physics engine simulates physics?"</strong></p>
  
  <p>Like lexer and parser are used to understand the code when compiling and ray-tracing is used to render a 3D scene, I think that there's some notions (other than collision detection) which are used in the physics engine to simulate physics, like calculating torque and velocity of a pentagon performing barrel roll.</p>
  
  <p>How does a physics engine actually simulate physics? What notions are used? Is there any 'tutorial' on the web about making physics engine like <a href="http://www.devmaster.net/articles/raytracing_series/part1.php">this one</a>, which demonstrates ray tracing?</p>