Explain how the arithmetic sum of the two forces can be greater than the load.

Force is a vector. If you have one force pointed northeast and another northwest the east/west component would cancel each other out. The arithmatic sum of their magnitudes is the whole force. If you have equal forces, one going north and one south, the net force would be 0 no matter how large the arithmetic sum might be.

Some type of thing that was invisible

If there was no gravity then their would be no downword force on the football to have it return the the grownd, and if there was no friction (mainly air friction) then there would be no opposing force the stop the ball from moving through the air allowing the ball to travle ∞.