Calculating mutiple forces on a body

[johnpauldisec]

Hi
I am making physics engine (computer) and i have some problems with interpretating multiple forces on one body.

http://img837.imageshack.us/img837/396/75i7i47i6.png

On the fourth car (on right) there are two forces acting on two rear wheels, interpretation is obvious, but what when only one force is acting ? Which of these three interpretation, if any, is correct?
How to calculate how much force is moving body and how much it is rotating it (moment) ?
I currently use second image interpretation but i think it is not correct.

And, second question, how to calculate when mutiple forces with random angles etc are present ?
I think somebody asked that question before: https://www.physicsforums.com/showthread.php?t=222598
but i don't understand answer he was given.
I have force angles and forces, but i can have x and y components about any axis/angle. I need a way to calulate resultant force/moment.

 

[mfb]

The total net force is always the (vector) sum of the individual forces, and you can sum all x-components for the x-component and all y-components for the y-component.
The total net torque is always the sum of all individual torques, you can simply sum them.
Diagram 2 looks good. Keep in mind that the rotation of a car is usually determined by the position of the wheels - external torque does not matter as long as the wheels do not slide.