From a physics standpoint, a bunt, like a swing, is a matter of momentum.
Anyone experiencing a car accident or a football tackle intuitively understands momentum: The bigger they are -- and the faster they move -- the harder they hit. As it happens, this is pretty much on-base, because the physics definition of linear momentum, p = mv, says that momentum (p) = mass (m) times velocity (v). Roughly speaking, mass measures inertia -- the tendency of a thing to resist changes in its state of motion -- while velocity refers to a combination of speed and direction.
Now, if you are prone to physics panic or allergic to algebra, don't stop reading just yet. Although a swing entails all kinds of variables -- from the batter's body dynamics to the swing's angular whip -- we can ignore most of these factors because the impact force between bat and ball is so much bigger [source: Fencl].
Let's take a closer look at that impact.
According to Newton's laws, barring outside interference, momentum in a collision -- such as the smashing of a bat into a ball -- is conserved. In other words, what goes in must come out. Often, some portion of momentum transfers from one object to the other in the process. In a successful swing, for example, so much of the momentum of the bat is transferred to the ball that its direction is reversed.
Most collisions are inelastic, meaning some energy is lost to deformation. Just as cars don't bounce off each other like hockey pucks, baseballs don't stay round during impact. Instead, they (and, to a lesser degree, bats) compress and rebound, resulting in an energy loss described by a coefficient of restitution (e) between 0 and 1, with lower values meaning more energy lost. Modern baseballs produce a restitution coefficient of around 0.55 for a 90 miles per hour (145 kilometers per hour) pitch [sources: Kirkpatrick; Manning; Russell].
The physics at play during a bunt are even simpler, but the goal is reversed. Where a slugger wants to tee off on the ball, a bunter tries to absorb its momentum. In fact, coaches often instruct bunters to "catch" the ball with the bat -- easier said than done when pitches can curve, slide, drop or scream in at better than 100 miles per hour (161 kilometers per hour). Bunters must carefully avoid popping up, rolling foul or back-spinning the ball such that it strikes the bat a second time (an automatic out) [sources: Short].
Batters will sometimes bunt with a stiffer bat, too. As we'll see in this next section, it all comes down to what they want to accomplish.