  We calculate the interaction potential between N atoms and NH molecules and
use it to investigate cold and ultracold collisions important for sympathetic
cooling. The ratio of elastic to inelastic cross sections is large over a wide
range of collision energy and magnetic field for most isotopic combinations, so
that sympathetic cooling of NH molecules by N atoms is a good prospect.
However, there are important effects due to a p-wave shape resonance that may
inhibit cooling in some cases. We show that scaling the reduced mass used in
the collision is approximately equivalent to scaling the interaction potential.
We then explore the dependence of the scattering properties on the reduced mass
and explain the resonant effects observed using angular-momentum-insensitive
quantum defect theory
