I’m confused about the difference between sweet spot and moment of inertia.
Are they the same?
The sweet spot is the impact point on the clubface where you will feel the least amount of clubhead twisting—maybe none—during or after impact. This spot generates maximum momentum transfer (and highest ball speed). Moment of inertia is the resistance to angular acceleration (twisting during impact). A high moment of inertia (MOI), or high resistance to twisting, is the result of distributing the weight in a clubhead as far away from the club’s center of gravity as possible, like in a hollow-headed driver or a heel-toe weighted putter. The Frankly Frog putter I designed has a high MOI and is very forgiving as a result of the weight distribution.
Impacts away from the sweet spot on a club with a high MOI will feel more solid than the same impacts on a club with a low MOI (where the clubhead will twist more during impact which you will feel in your hands after impact).
However, saying a high-MOI club has a larger sweet spot isn’t true. All sweet spots are the same because they are a single point. MOI can be different, so the area of the clubface yielding a sweeter feeling can be larger on one club than another (even if the clubheads are the same size), based on how the weight is distributed. But hitting the sweet spot with the driver doesn’t always result in maximum distance. You actually get more distance by slightly compromising ball speed for a higher, lower spinning launch angle by contacting the ball higher on the clubface.