FixedUpdate and how to physics?

Since FixedUpdate uses a fixed time, it means if the time was changed it would break the physics. So if I add 10 force per step and then change fixed update rate to be twice, it will be adding the force twice as fast, which is not good.
Any tips to prevent this from happening? Maybe something like deltaTime which more or less smooths things out no matter how fast Update() is called. Just not changing the timestep is not really a solution, just a weak glue based on hopes and dreams that there will never be a need to change the step.

Thanks

 

http://docs.unity3d.com/Documentation/ScriptReference/Time-fixedDeltaTime.html

forcePerSecond * Time.fixedDeltaTime instead.