I am a beginner and this is my first post. We know torque ## \vec T = \vec r × \vec F ## Now, should r be fixed in magnitude? I mean can I calculate torque only if the particle is rotating at a fixed distance around a fixed axis? I am having this idea because torque is explained (from where I read) using an analogy with doors. Second question. I read that generally a system of forces acting on a rigid body can not be reduced to a single force equal to the vector sum of the forces. Then I get confused because in case of weight, we replace gravitational forces on each particle of a rigid body with a resultant force through the center of mass. Will someone please explain?