When wind applies a force to a pinwheel, the arms of the pinwheel start to move in a circular motion. Which additional force keeps the arms of the pinwheel moving in a circular motion?

The reaction force from the shaft, which keeps the wheels in place during operational rotation

Explanation:

The pinwheel design which has the same principle of a turbine, relies on a external energy source such as the wind or water to provide motive force for spinning

Whereby the reaction force provided by the shaft holding the wheel is non existent or insufficient, the wind will be able to push the wheel in its direction thereby giving the wheel only a net translation and no rotation.