6.19 FRICTION

Let's imagine a situation where a force affects an object. The force accelerates the object according to the Newton's law of motion: F=ma. This means that the longer the force affects to the object, the faster the object moves. However, this is not what appears to happen in the real world. The reason for this is that usually there are some other forces involved, making it very difficult to generate very fast motions. For example, the air is a such a resisting element. These resistive forces are called friction forces.

The FRICTION method can be used for adding these "resistive" forces to the virtual reality of REAL 3D. If the velocity of an object is 0, the friction method does not affect it at all. The higher the velocity of the object, the more the friction affects it, direction of the friction force being always opposite to the velocity of the object. The strength of the resisting force is proportional to the velocity squared.

The strength of the force depends on the size of the object, too. The bigger the object, the higher the friction force. However, the size has less dramatic effect to the force than the velocity attribute; the strength of the force is increased only linearly proportionally to the size.

The mass of the object naturally determines how much the friction force can affect its velocity. The higher the mass, the less sensitive the object is to the friction force.

The strength of the force generated by the friction method is defined using the following formula:

                              d * e˛
                   f = Fmax * ---------
                              d * e˛+ 1

where d is the diameter and e is a total energy (kinetic+spin) of the object. The Fmax is the force needed to stop the object during the time interval between two subsequent frames. The formula can never produce bigger forces than this maximum force.

The friction method does not require parameters.