The animation system AERO is presented in this paper. It allows the creation of virtual environments from scenes with simple three-dimensional geometrical objects (sphere, cylinder, box, point, plane). Objects can be linked to each other by a variety of methods (rod, spring, damper, joint). Realistic object movements are achieved by a simulation procedure, where forces can be applied to the objects in addition to gravity, friction, and air resistance. Collisions between objects are simulated using either the "penalty method" or the änalytical method", described in the text. Wire frame animations can be generated in real time, depending on scene complexity and computer system performance. AERO also generates scene description sequences, which can be used as input for ray tracing programs to generate photorealistic animations as batch jobs, a task that is also called "physically based modelling". AERO has a number of special features, such as stereo graphics output, camera control, and even the mounting of the camera to an object in the scene.