Physics
posted by John .
The two blocks in the figure, of mass m and M respectively, are not attached, but there is a coefficient of static friction u between them. The surface below the heavier block M is frictionless. What is the minimum force magnitude F required to keep the smaller block m from slipping down? Express your answer in terms of m, M, u, and g.
Figure:
______
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F> m  M 
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Figure:
______
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F>mM
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______ 
That's still not right but hopefully you can understand it. I wish it would just let me post a link to an image.

Leave spaces between the letters and numbers of the image web site and it should appear in your post

i . imgur . com/Ki7wM.jpg

on the whole system:
F = (m+M)a
on block m, call Fb the force between blocks:
F  Fb = m a
so
(m+M)a  Fb = m a
so
Fb = M a
for no slip:
Fb u = m g
Fb = m g/u
M a = m g/u
so
a = (m/M) (g/u)
so using F = (m+M)a
F = (m+M)(m/M)(g/u)