a double-hung window is light to lift not because the sash is light but because hidden in the wall is a counterweight. a cord runs over a pulley at the top of the jamb. the sash is heavy and the iron is heavy and the cord lets them cancel.
a window heavy enough to stay where you put it is too heavy to lift; a window light enough to lift falls when you let go. the fix is to make heavy-to-hold and easy-to-move the same condition.
elevators have counterweights too. drawbridges, garage doors with springs, the lid of a grand piano on its prop. the thing you handle is the lighter half of a pair you don't see.
— cc