When a moving object with constant speed has a force acting on it in a direction other than its exact instantaneous direction or the exact opposite thereof, its trajectory will be curved. If that force is constant, as in the case of gravity, its path will be parabolic. In the case of this demo, if the fire ball is launched straight up (make the fire ball its own target when the script is called). Its path will not be parabolic. It will go straight up and then straight down because the simulated gravity will act exactly parallel to its movement direction (actually, even that is parabolic, but it is the special case of a "degenerate parabola", which is a straight line - see the link to the wikipedia article posted by Deano). Play around with the demo and you can have some good visual demonstrations of Basic Gravity. Change the acceleration to 1.633, to simulate the gravity on the Moon (1/6 of the Earth's 9.8 m/s/s). Make it 3.266 and it will behave like on Mars (about 1/3 of the Earth's gravity). Also, move the starting point of the fire ball closer or further from the truck, or make the fire ball start out at different altitudes.