Descriptor of the force acting on the object.
It is given in the form F=<t,fn,ft>, where:
- t: double that identifies the location on the objet's perimeter where F is applied.
- fn: double that specifies the amount of force applied along the direction normal to the object's perimeter; fn>0 for inward direction.
- ft: double that specifies the amount of force applied along the direction tangential to the object's perimeter; ft>0 for direction along velocity.