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SI (Le Système International d’Unités) was designed so that most
of the units are consistent with each other, and consistent with
the laws of physics. For example, the SI unit of force is equal
to the unit of mass times the unit of acceleration, so we can
write F=ma without any conversion factors.
Other systems of units are not always so consistent, in which
case you must re-think many of the laws of physics, with an eye
toward putting in conversion factors where needed.
For example,
* If you want to measure acceleration in feet per second per
second, mass in lb, and force in lbf, then you cannot write Newton’s
second law in the form F = m a. Instead you must write F = m a/g.
This is what I usually do, on the rare occasions when I am forced
to calculate with pounds at all.
* If you want to measure mass in lb and force in lbf, and still
write F = m a without a conversion factor, then you must measure
acceleration in Gees (not in feet per second squared). This is
almost equivalent to the previous option; it just absorbs the
conversion factor into the definition of acceleration.