Here’s How to Crack It
The center of mass of the bar alone is at its midpoint (because it is uniform), so we
can treat the total mass of the bar as being concentrated at its midpoint.
Constructing a coordinate system with this point as the origin, we now have three
objects: one with mass 5 at (−9, −6), one with mass 10 at (9, −12), and one with
mass 15 at (0, 0).
Choice of Origin
Remember, you can
choose your origin
anywhere, but a good
strategy is to use the
location mentioned in the
problem. If it asks, “How
far from the left is the
center of mass?” choose
the left as your origin.