Whe hanging up curtains, you have to do work on them. The screws in the walls act as. Normal force against gravity. If your curtans are too heavy and your rod isn't strng enough material, gravity may cause a bend in the middle of the rod because the force of gravity is stronger than the stiffness of the rod. Be sure your rod isn't made ofamber because it can cause electrostatic force it rubbed up against the curtain.