Answer:
The answer is chlorophyll.
The activation energy of a given chemical reaction is the minimum required amount of energy needed to start the reaction, for the reaction to start. All reactions have an activation energy, even spontaneous ones, ones that do occur spontaneously. It is a thermodynamic barrier that separates the amount of energy reactant molecules must possess to ultimately react with each other to produce the respective products.
"Net force" simply means the sum of all forces. The "unless acted upon by a net force" version is more correct. For example, as you stand or sit still, the earth pushes you up with a force equal and opposite to the force with which gravity pulls you down. The net force, or total force, is zero, so you do not move.
