Final answer:
The reaction force to a bat hitting a baseball is the force that the baseball exerts on the bat, which is equal in magnitude and opposite in direction to the action force applied by the bat onto the baseball.
Step-by-step explanation:
When considering the action force of a bat hitting a baseball, according to Newton's third law of motion, there must be a reaction force that is equal in magnitude and opposite in direction. This interaction is an example of contact forces, which are explained by the charge interactions between atoms and molecules in close proximity, such as the Coulomb force.
The reaction force in this scenario would be the force exerted by the baseball onto the bat. During the impact of a bat with a baseball, both objects exert forces on each other, leading to the mentioned action-reaction pair. This principle is observed in various other scenarios, such as the feeling of recoil when swinging a bat, which is the baseball exerting a force on the bat, or the backward force felt from a discharged rifle, which is equal to the force propelling the bullet forward.