Friction between objects

1. The problem statement, all variables and given/known data

Please see attached

2. Relevant equations

Ff=μFn,
F=ma

3. The attempt at a solution
(a) The blocks can have the same acceleration due to static friction.
consider the 2 blocks on the left: Ff=fmg

then consider the 2 blocks on the right: Ff is also fmg? I am rather confused at this point... how does M come into the equation? because the answer for part (a) is 2(M+m)/(M+2m)f. There is no g anywhere in the equation, though from the equation, friction is to the normal, which is mg?

THank you!

 

yeah okay, but how to do the question? Can give me a head start? Thanks!

 

sorry, I don't really get what you mean, please elaborate... So for the small mass on the right, f value must be (m+M)f?

 

Okay, I think I know...

So the whole thing F=2(M+m)a .........(1) (as stated by azizlwl)
the greatest friction is between the blocks on the left, so fs=(2m+M)a ........(2) (thanks azizlwl!), because the bottom block on the right need not be considered. So F/fs=2(M+m)/(2m+M), and F=2(M+m)f/(2m+M)

Then for the 2nd part, F=2(M+m)a .........(1) still,
then f is only for the block M on the right, thus f=Ma,
by the same reasoning, F=2(M+m)f/M.

Yay thanks guys! Any tips for force analysis (and consider which few objects as a system?)