What is moment of inertia
"Moment of Inertia," or MOI, is a term in physics that indicates the relative difference in how easy or difficult it will be to set any object in motion about a defined axis of rotation. The higher the MOI of an object, the more force will have to be applied to set that object in a rotational motion. Conversely, the lower the MOI, the less force needed to make the object rotate about an axis.
How does this low or high MOI in a club head help the golfer?
When you hit a shot off the centre of the club face, the club head will rotate in the opposite direction around its vertical axis. For example, a shot hit from the toe of the club head will have the club head opening up and pointing to the right side of target through impact. Now, the higher the MOI in the club head, the less will be the opening or closing of the club head due to an off centre hit and the more will be the resistance to this opening or closing. Golf club manufacturers call this the amount of forgiveness a club head offers for off - centre strikes.
The larger the club head and the more the manufacturer incorporates perimeter weighting around the club head, the higher the MOI of the club head about its centre of gravity and vertical axis will be. The higher the MOI of the head about its vertical CG axis, the less the head will twist in response to an off-centre hit, and the less distance will be lost from that off-centre hit.
The smaller the head and the more head weight is positioned close to the centre of the head, the lower the MOI of the head, and the more distance will be lost when the ball is hit off centre.
Again, higher MOI = more resistance to the object being rotated around an axis; lower MOI = less resistance to the object rotating around an axis.