For every action there is an equal and opposite reaction. Basically gravity is the external force which can (and does) change the motion of an object; recall ‘gravitational constant’.

Torque = force x radius.

If the force applied is constant but say the wrench is longer, then the torque generated will be more, and vice versa.

When you use a wrench with a preset amount of torque, it tightens the screw to that torque value and in order to loosen it up you will need to generate a minimum amount of that torque in the opposite direction.

The purpose of tightening screws/fittings to a specific torque value is so the thread does not get damaged; especially when you need to undo/redo them often.