A sled loaded with firewood is pulled by a tractor. The tractor exerts a constant force with magnitude 2000 N in the horizontal direction. The total weight of the sled and the firewood is 4000 N. What is the net force acting on the sled and firewood system?
1) 2000 N
2) 4000 N
3) 6000 N
4) 8000 N