For effective communition in a ball mill, it is desired that the balls travelling along the mill wall leave the wall at point C and travel freely in air along the path CDA, as shown in the figure. If ∠BOC is 120°, the rotational speed of the mill is _______ rpm (rounded off to one decimal place) by performing suitable force balance at point C.
Assume:
1. There is no slip between the ball and mill wall.
2. O is the rotational axis of the mill and OB is parallel to the vector g.
3. Inner diameter of ball mill is 3.26 m.
4. Acceleration due to gravity g is 9.8 m⋅s⁻².