<p>The sport of baseball contains many interesting and intricate fluid mechanics problems. One of the more interesting ones is the effect of the rotation of the ball on the trajectory of a pitch. Sliders, fastballs, curveballs, etcetera all take advantage of the Magnus effect to pull the ball away from the straight-line path to which it is delivered. The Magnus effect is a well-known and well-studied phenomenon for spheres. The baseball problem is complicated by the existence of protruding seams on the surface. While the Magnus effect typically results from varying shear forces on opposite hemispheres causing different separation points on either side of the ball, the seams also act as boundary layer trips which can create their own separation point for the flow under certain conditions and orientations. This study models the transient flow over a baseball as it rotates. An unsteady RANS simulation was conducted using the realizable k-epsilon model with scalable wall functions. The variation of lift and drag forces experienced by the baseball as the seams rotate during the pitch were quantified. The current study shows that for a four-seam fastball, the lift force can vary by 20% from the mean, while the drag force varies 10%. The variations occur as the seams change position and advance the separation point of the boundary layers on the top and bottom of the baseball. The fluctuation of the aerodynamic forces calculated for the rotating ball are significantly less than those measured by studies in which the seam location is varied on a non-rotating ball. By better understanding the physical phenomena which account for these variations, manipulations may be possible to affect the success of the pitch.</p>