text: The man is pushing against the box in one direction, and against the floor (with an equal force) in the opposite direction. The force on the floor is transmitted through his shoes, via a frictional force. And the man experiences reaction forces in his hands and feet (the box and the floor "pushing back")\n\nWhen stationary, or at constant velocity, all the forces balance. As the box starts to move (accelerate), there must be a resultant force in the forward direction, i.e. the forces are not balanced.