How can I keep the smaller block from slipping down the larger block?

[oscarkool2]

Homework Statement

The two blocks (m = 14 kg and M = 100 kg) in Fig. 6-39 are not attached to each other. The coefficient of static friction between the blocks is μs = 0.28, but the surface beneath the larger block is frictionless. What is the minimum magnitude of the horizontal force required to keep the smaller block from slipping down the larger block?

Homework Equations

no idea

The Attempt at a Solution

Couldn't attempt anything because I don't know where to began and the horrible Haliday text doesn't help me.

 

[AEM]

Can you describe the orientation of the blocks a little better. Are they on the level or on an incline? What do you mean by the smaller block slipping down the larger block?

 

[oscarkool2]

its a flat surface with block M on the gound at rest and block m pushing against it, but block m isn't on the ground. the friction is between block M and m. its pushing against M on the left side. I have no idea why my teacher assigns us to do homework when his lectures never cover any examples like these problems he gives us...

 

[AEM]

its a flat surface with block M on the gound at rest and block m pushing against it, but block m isn't on the ground. the friction is between block M and m. its pushing against M on the left side. I have no idea why my teacher assigns us to do homework when his lectures never cover any examples like these problems he gives us...

Ahh, what a cool problem! Draw yourself a picture of just the small block and identify the force on it. Two questions: (1) What force keeps the block from falling? (2) What is the equation that tells you the magnitude of that force?

Answer those questions and I'll give you the next hint.