#2.8 "A 4-kg block rests on top of a 5-kg block, which rests on a
frictionless table. The co-efficient of friction between the blocks is
such that the blocks start to slip when the horizontal force F applied
to the bottom block is 27 N. Suppose the horizontal force is now
applied only to the upper block. What is it's maximum value for the
blocks to slide without slipping relative to each other?"
Since the only contact surface with friction is that between the two
blocks and the normal force on this surface is the weight of the top
block, the force for slippage should be the same regardless of whether
it is applied to the top block or the bottom block. The answer given
in the book is 21.6 N. What am I missing?