Force is related to Quantity. Constant is a type of Quantity. 
Photography is a type of Process. Process is related to Thought. Mainstream is a type of Thought. Mainstream is a synonym of Usual. 
Force <--RelatedTo--> Quantity <--IsA-- Constant 
Photography --IsA--> Process <--RelatedTo--> Thought <--IsA-- Mainstream <--Synonym--> Usual 