Mastering something means understanding how it works, inside out. However, modern game engines like Unity allow to use very complex technology apparently without the need to understand the inherent details. 3D games that want to simulate any aspect of reality have necessarily to simulate the physical world we live in, together with its laws, and for this reason, any game developer ought to have at least a basic grasp of math, geometry, and physics. This chapter discusses Newtonian physics and shows how to implement a simple physics engine to recreate proper rigid body behaviors within Unity itself. It explains the application of Newton's three laws of motion within Unity.