Today in our Physics class, we learned about static equilibrium of solid rigid. The teacher presented us with an example in which a man was walking up an inclined board held up by the ground on one end and by his truck on the other (I hope this picture makes sense).
At any given moment, the teacher represented all the forces that were acting on the board: its weight, the vertical normal force from the ground,the horizontal normal force from the truck, and then the (vertical) weight of the man on the board.
Can someone please explain to me why we consider that the force the man exerts on the board is its weight and not the reaction (Newton's 3rd law) to the normal force the board exerts on the man? What would happen if instead of a man we had a ball?
I am really confused and any help would be really appreciated.