It's Basic Physics

When looking at a collision between a baseball bat and ball, three things always apply:

Conservation of linear momentum-

The linear momentum of a particle of mass, m, moving with a velocity, v, is defined to be the product of the mass and velocity: p=mv

Elastic collision-

An elastic collision between two objects is one in which total kinetic energy (as well as total momentum) is the same before and after the collision.

Conservation of energy-

Energy can never be created or destroyed. Energy may be transformed from one form to another, but the total energy of an isolated system is always constant.

Newton's Third Law-

States that for every action there is an opposite and equal reaction.

How hitting the ball works

The Swing

When a person swings a bat, their arms propel the bat to a high velocity which is needed to transfer momentum to the ball and send it sailing. Also, there is a transfer of energy starting with the batter then moving from the batter's arms to the bat and then when contact is being made with the ball the energy in the bat is transferred to the ball which propels the ball forward (hopefully:)) The force that acts on the ball, as contact is being made (contact is about 1/1000sec. long), can easily reach a couple thousand pounds.