Shoot a cannonball horizontally from a cannon at a mountain top and it gains speed as it falls and ...

 The slower cannonball has a parabolic trajectory with a component of force in its direction of motion. This increases the speed of the falling cannonball. When it is fired in circular orbit, the trajectory is circular and the force is perpendicular all along the path with no component of force in the direction of motion. Put another way, in circular orbit the force of gravity does no work on the cannonball. Therefore there is no change in speed. When fired at a lower speed, where the cannonball arcs downward, a component of force in the direction of motion exists. Then work is done and the cannonball gains kinetic energy.