The Physics of Baseball Bats

Aspiring baseball players across the nation are preparing for the spring season to begin and many of them are freshmen about to try out for their high school team. This time is a stressful one where dreams are achieved or broken and many players are shopping around for new gear to aid their performance. In my opinion, the most important choice one can make about their baseball gear is the bat they decide to purchase; while the majority of bats generally serve the same purpose, the construction of the bat can either enhance or hinder the ability of the player. Under the national BBCOR league, bats that are created by companies must perform about equivalently to the wooden bats seen in the MLB; however, clever engineers have discerned a way to tailor a bat to the specific needs of the player. Many baseball sites will display a graph similar to the one below:

This chart is an incredibly simplified explanation about the moment of inertia (MOI) of the bats a site has for sale. A baseball player transitioning from another league can use this chart to decide which baseball bat would be best for them. The numbers in the chart inform the player how "heavy" the bat will feel to swing; heavy is in quotes because, as a BBCOR standard, all bats will be -3. This means that the bat will weight (in ounces) three less than the length of the bat (in inches). Therefore, every 33" bat will weigh 30 oz. If all the bats weigh the same, how do the numbers in the chart come into play? The answer lies in physics.

The distribution of mass throughout the length of the bat is the distinctive feature demonstrated in the chart. The Mass Index shown above gives the player an indication of the MOI of the bats. For example, a lower number (650-700) means that more of the mass of the bat is positioned closer to the player's hands. This diminishes the distance the mass is from the axis of rotation, making the bat feel lighter and making it easier to swing. A higher number (950-1100) means that more of the mass is positioned near the endcap of the bat (the furthest position on the bat from the player). This makes the bat feel heavier and making the bat more difficult to swing. Younger and smaller players that are not strong enough to effectively use an "end-loaded" bat would benefit from the bats with a lower Mass Index (MOI) to create adequate bat speed through the zone to make consistent contact with the pitch. Conversely, a stronger, more developed player that can swing a bat quickly through the zone can use a bat with a higher Mass Index to increase the angular momentum behind their swing, giving them more power and launching the ball further.