Physics, two masses on eachother, maximum horizontal force?

Question

A 4.15 kg rabbit is on top of a 5.23 kg box. A horizontal force of at least 12.1 N must be applied to the rabbit to cause him to slip on the box (which is held stationary). If the rabbit-box system is placed on a frictionless floor, what is the maximum horizontal force that can be applied to the box so that the rabbit and box move together (ie. the rabbit doesn't slip). Any help would be appreciated. Thanks

Answer 1

Hor. accel for the rabbit to slip As = fslip/mrabbit.
Max force applies this accel to mbox + mrabbit:
fmax = As*(mbox + mrabbit) = (fslip/mrabbit)*(mbox + mrabbit) = fslip*(1+mbox/mrabbit)