How does baseball interact with newtons laws.

Newtons 2nd Law- The Net Force (ΣF) that is applied to an object is equal to its mass (m) multiplied by its acceleration (a).
F=m*a

Just like in Newton's first law above, the hips again are causing the force.  The acceleration is generated from the hips motion.  To obtain the largest amount of force, the player would need to be able to accelerate his hips along with having a heavy mass(the bat) to strike the ball.

Instead of linear motion that generates the force, a baseball swing is calculated using rotational motion. 
 
"Rotational motion is similar to circular motion, except the object involved is a rigid body in which all points rotate around the center of mass of the object and not around a fixed point". (2)

 Newtons second law still applies, but the force equation is transferred into rotation force, called torque.  The mass is replaced with a quantity called inertia and the acceleration is now called angular acceleration.     

T(torque)= I(inertia)*α(angular acceleration)

I=ß(constant)*m(mass)R(radius)^2

α=a(linear acceleration)/R(radius)

The torque then is dependent on the radius.  the smaller the radius is, the larger the angular acceleration.  When a player lowers the radius of his bat, he will have a smaller moment of inertia and a larger acceleration, maximizing his torque.