This app represents the equivalence principle which states that gravitational and inertial forces are of a similar nature and often indistinguishable. Left rocket is moving up with constant acceleration a, and right rocket is on the some planet where the gravitational acceleration is g. If a is equal to g, the motion of a ball which is dropped in the left rocket is the same as the motion of a ball which is dropped in the right rocket; two balls experience the same constant acceleration motion. This is the simple demonstration of the equivalence principle. This app doesnt simulate the whole motion of a ball. Only falling motion is intended to show the equivalence principle.