Friction pinning a block on another block

[richard karn]

Homework Statement

The two blocks (m = 36 kg and M = 98 kg) in the figure below are not attached to each other. The coefficient of static friction between the blocks is µs = 0.48, but the surface beneath the larger block is frictionless. What is the minimum magnitude of the horizontal force vector F required to keep the smaller block from slipping down the larger block?

 

[The legend]

hmm, where's the figure? and also you are supposed to post an attempt to the solution..