I'm trying to make a superhero that becomes as strong as he needs to be for whatever situation he's in; he supplies a reaction equal to whatever's acting on him. So if a car was coming at him with the force of x Newtons, this guy becomes strong enough to supply an opposing force of x Newtons and stop the car.
The thing is, the power works for stopping objects, but how would it work if he wanted to punch someone? It doesn't come under the same umbrella as Newton's Third Law, and he's essentially no longer supplying a normal force, and different punches are going to have different effects. I don't really want this guy to have a choice in how strong he gets - it's totally automatic. He's more immovable object than unstoppable force; would he still be able to fight properly?
Any ideas as to how he would work in a fight, or should he just be defensive?