转动惯量是定量描述刚体绕定轴转动时惯性大小的物理量。它与刚体的质量分布及转轴的位置有关。对于质点而言，由于它只有平动，没有转动，因此说质点的转动惯量是没有意义的。不是刚体（比如流体），不存在转动惯量。

The moment of inertia is a physical quantity that quantitatively describes the inertia of a rigid body rotating around a fixed axis.It is related to the mass distribution of the rigid body and the position of the rotating axis.For a particle, since it has only translational motion and no rotation, it makes no sense to say the moment of inertia of the particle.It's not a rigid body (such as a fluid), there's no moment of inertia.