# Moment-of-inertia $I$

How “tough” it is to turn something (to slow down or speed up that turning). Rotational equivalent to mass.

$$I=\sum mr^2$$

$m$ is mass and $r$ distance from the axis-of-rotation to each particle of the rotating object. $I$ is different about different axes.

• Scalar property
• Symbol: $I$
• SI-unit: Kilogram-metres-squared $\mathrm{kg\cdot m^2}$

Moments-of-inertia about various axes are found in Resource: Moments-of-inertia.