When you exert a force on a baseball, there exists an equal and opposite force on the ball therefore, the ball will accelerate in opposite direction.
Explanation:
When you hit a ball with baseball bat, the bat exerts a great force on the ball which causes the ball to accelerate in the opposite direction. It is to be noted that the mass of bat is much greater than mass of ball but the acceleration of ball is also greater than the acceleration of the bat so both bat and ball almost exert same magnitude of force but in opposite direction and as a result both bat and ball accelerate in opposite direction, the deciding factor is of course the relative force applied by the batter and the bowler.
