What is the angular momentum of a rod with a mass of #2 kg# and length of #9 m# that is spinning around its center at #12 Hz#?
The angular momentum is given by
For a thin rod with uniform density rotating about the centre, the moment of inertia is given by
You can find moment of inertia formulas for different shaped objects tabulated. In this case
We want to work in SI units, so convert the frequency to angular velocity using
This arises because each rotation is
Finally, substitute these values into the equation to calculate the angular momentum
Notice that we ditch the radians in the units. This is a a thing that I don't really understand but is due to the definition of the radian being a ratio of two lengths (so units cancel out). It gets used and dropped sporadically.
Careful with significant figures too.