Angular momentum can change whenever an object experiences a change in angular speed, a change in direction of rotation, or a change in rotational inertia, which could involve a change in mass and/or mass distribution/size.

The change in angular momentum ($\Delta \vec{L}$) is the difference between the final angular momentum $\vec{L}_f$ and the initial angular momentum $\vec{L}_i$ of an object over some time interval.

For a rigid object with constant rotational inertia $I$, that is to say constant mass and size, the change in angular momentum is directly related to the change in angular velocity $\Delta \vec{\omega}$.

$$\Delta \vec{L}=I\vec{\omega}_f-I\vec{\omega}_i=I\big(\vec{\omega}_f-\vec{\omega}_i\big)=I\Delta \vec{\omega}$$

The magnitude of change in angular momentum $|\Delta \vec{L}|$ depends on several scenarios:

If the object speeds up while rotating in the same direction, then $|\omega_f| > |\omega_i|$ and $|\Delta L| = I(\omega_f - \omega_i)$.

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