Let's say you put a box on a board, like this. (the dot represents a fulcrum point). Let's say the center of mass is exactly halfway between the fulcrum and the edge of the board. Now let's say you exert an upward force F in the direction of the arrow. My book tells me that the force should be reduced by a factor of 2.. which I suppose is plausible, but how can you know the exact mechanical advantage without knowing the length of the board?