Keep mass vertically still using friction

[knc]

A mass m1 is on a frictionless surface. To its right is a second mass m2 not touching the ground, but in the air. An applied force is applied towards the right of both masses. What is the minimum force needed so that the second object JUST starts to slip downwards?

http://i.stack.imgur.com/gQNg8.jpg

I have no idea where to start... I know I need to find the normal force but I have no acceleration.

 

[CWatters]

Write an expression for the acceleration in terms of other parameters.