Friction with moving block question

[ethrust2]

The drawing shows a large cube (mass = 49 kg) being accelerated across a horizontal frictionless surface by a horizontal force P. A small cube (mass = 4.6 kg) is in contact with the front surface of the large cube and will slide downward unless P is sufficiently large. The coefficient of static friction between the cubes is 0.71. What is the smallest magnitude that P can have in order to keep the small cube from sliding downward?

http://www.flickr.com/photos/42276194@N04/3981264713/

Anyone have any idea?

I tried both normal forces multiplied by the static friction coefficient.

 

[tiny-tim]

Welcome to PF!

Hi ethrust2! Welcome to PF!

Hint: for a particular P, what is the acceleration of the small block?

and so what is the reaction force on the small block? ​