rotational dynamics - What rotates a door : torque or centripetal force?

I can realize that they are different, but can't understand what actually creates a rotational motion. Rotational motion doesn't mean just moving in a circle. There must be rotation of the position vector for rotational motion. For that, torque is required. Centripetal force,on the other hand,rotates the velocity vector. If this is the difference,what actually makes a door to rotate? Most book writes, it is torque, since the postition vector of the door's edge is rotating with respect to a hinge. But isn't there centripetal force,which is changing the direction of the velocity vector of the edge of the door to let it move in a circle?? What is actually happening? What is the difference between these two (apart from units!)??

Answer

You need a torque/force to start or stop a door, because an external torque makes something slow down how fast it rotates or speed up how fast it rotates or change the direction of how it rotates.

But an external torque is not why the door keeps rotating. Imagine that you and your friend are out of the ice and you grasp each other hands facing each other. If a torque gets you spinning (say someone/something pushes your friend east and pushes you west) then you have hold on to each other to rotate in that symmetric non-expanding way. This is how rigid objects rotate, they are holding themselves together because they are stretching like a rubber band.

So let's now look at a top. At rest it is a certain size, the size that is a natural equilibrium based on the stuff it is made of of, how far the atoms can be and hold each other in place. But if you spin it, then it gets fatter in the middle, but that equilibrium position resists you making it bigger so it tries to pull itself back together, but the direction it pulls itself together is orthogonal to the velocity, so it doesn't change the speed, so it just keeps spinning. This force is a force that the object is exerting on itself. The parts on the outside are pulling the inner parts in an outward direction while those inner parts are pulling the outer parts in an inwards direction, but nobody goes any faster or slower because the directions they are pulling are orthogonal to the velocity.