For positive constants a and b the force f between two

Question

Question: For positive constants A and B, the force, F, between two atoms in a molecule at a distance r apart is given by

F = (-A/r²) + (b/r³)

(a) How fast does force change as r increases? What type of units does it have?

(b) If at some time t the distance is changing at a rate k, at what rate is the force changing with time? What type of units does this rate of change have?