Determine the angle alpha in the context of pulleys A and B with given radii and distance between centers, as part of a belt system calculation.

The radius of pulley A is $R_A = 21$ cm.
The radius of pulley B is $R_B = 10.5$ cm.
The distance between the centers A and B is $d = 40$ cm.

Draw a line from the center of the smaller pulley (B) parallel to the straight belt segment to the radius of the larger pulley (A). This forms a right-angled triangle with vertices A, B, and a point C on the radius of pulley A.
The length of the side $AC = R_A - R_B = 21 - 10.5 = 10.