"strength" is a person's property or quality. "Force" is a physical abstract power which has a physical effect within the physical world.
Thus "strength" is a quality of the body which enables you to applie a force to the outside world.
With his strength he could push the car with some force.
The term "force" as you might know is also applied to military troops. The reason for this is the image of war as the almost physical clash of the strength of two peoples.