Need a setup for a fiction problem

[Kadaj]

Need a setup for a friction problem

I know the concept of this problem but i don't know how to set it up...

'A large cube (mass= 25 kg) being accelerated across a horizontal frictionless surface by a horizontal force P. A small cube (mass= 4.0 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 0.17. What is the smallest magnitude that P can have in order to keep the small cube from sliding downward?'

Thanks in advance

 

[Kadaj]

anyone please help me

 

[faust9]

P of the box needs to equal the normal force of the little box onto the big box. Draw two FBDs. On of a little box showing the effects of friction, a normal force and weight. Remember, the little box is in static equalibrium when referenced to the big box. The entire two box system may be accelerating due to force P but the comparison of the two boxs should help you figure this out.

Draw a FBD of the big box as well. The N of the little box onto the big one equals the N of the big box onto the little BTW.

Good luck.