This Demonstration shows how a planet moves around a massive star like the Sun. Set the initial position and velocity before triggering the animation.
For the sake of simplicity, it is assumed that the gravitational constant is equal to unity and that the mass of the planet is much smaller than that of the star. You can view the instantaneous velocity and force acting on the planet (using arbitrary units) by checking the appropriate controls. The trajectory of motion changes as you change the initial conditions.