Simon created a physics engine in Python with Turtle. He used *Verlet integration* (French pronunciation: [vɛʁˈlɛ]), a numerical method for integrating Newton’s equations of motion in calculating trajectories of particles in molecular dynamics simulations and computer graphics.

Verlet Integration is a way to implement a physics engine without having to care about velocity.

Instead of storing the velocity, you store the previous position, and you calculate the velocity on the fly. Then if you add that velocity to the current position, you get the new position. But then you also have to add on the acceleration, because acceleration changes velocity.