can be distributed – centered and end loaded. Even though two bats (one more centered loaded and one end loaded) both weigh 30oz when put on a scale, their swing weights are much different which is very important to BBS. It takes more effort to initiate movement of the 30oz end loaded bat. It also becomes much heavier once a player initiates his swing. So just looking at the weight of the bat really doesn’t tell you how “heavy” it really is when it counts – the swing.
Dr. Daniel Russell (Professor of Acoustics & Director of Distance Education at Penn State University, who has done research involving structural vibration of sports equipment) states “There is a big problem with the discussion of bat weigh. All the physics used to derive the optimum mass and the batted ball speed assume that the ball hits the bat at its center-of-mass. This very rarely happens - hits at the sweet spot are several inches from the center-of-mass. There is another very important parameter of the bat which affects how quickly you can swing a bat, and what the final ball speed (BBS) is. This parameter involves the distribution of mass along the length of the bat and how that mass distribution affects the motion of a rotating object. In physics we refer to this parameter as the moment of inertia. It turns out that the moment-of-inertia (or "swing weight") matters more than mass.”
Moment of Inertia (MOI) – Swing Weight
Dr. Russell continues, “There are really only two things that matter – Bat Swing Speed and the Collision Efficiency between the bat and ball.” But both bat swing speed and the collision efficiency are affected by the Moment of Inertia (MOI) - the most important thing in creating optimal BBS.
Bat swing speed is self-explanatory. It depends on the player who is swinging the bat – their strength, mechanics, etc.
Collision efficiency depends on what bat the player chooses to create the optimum Force when the bat and ball make contact.
Moment of Inertia (MOI) (Just a little physics). Inertia is the tendency of a still object to remain still, and a moving object to keep moving. The Moment of Inertia is the instance when Force is applied to move the object. Therefore, a heavy object requires a great deal of effort (Force) to accelerate into motion from a standstill and will take longer to get to optimal bat speed. It also takes a longer time and distance to stop. The lighter object requires less effort to accelerate into motion from a standstill therefore will take a shorter time to get to optimal bat speed and it takes a shorter time and distance to stop.
“This idea of the moment of inertia has to do with not only the weight of the bat – but where the weight is distributed in the bat”, states Dr. Russell. Unfortunately,” he continues, “manufacturers don’t market MOI. Everybody knows (the manufacturers) that the MOI is what matters and that mass (the weight) really doesn’t matter nearly as much at all. When you buy tennis racquets or golf clubs you can purchase them by their MOI. Those industries understand that is what really matters”
But there is a catch. Too light of a bat (lower MOI) will have higher bat speed but will hit the ball less effectively. The flip side is that if a batter uses too heavy a bat (large MOI) he will hit the ball more effectively but will have a lower bat speed.
The Baseball Observer - Jan/ Feb 2016
46