Calculating Force F Required for Frictionless Blocks to Remain at Rest

[annatar]

Homework Statement

What is the magnitude of force F (which is pushing from the left on M) required to keep M1 and M2 relatively at rest to M?

Ignore all friction.

See attached diagram

Homework Equations

Newton's laws

The Attempt at a Solution

Well...If M is at rest, M1 and M2 will have an acceleration due to M2's weight.
When force F appears though, M2 will be pushed horizontally by M since they are in contact.

That's all I know atm.

 

[Astronuc]

The question asks about a force on M that allows "M1 and M2 to remain relatively at rest to M".

So what is the condition between M1 and M, such that M1 stays at rest with respect to M? If M1 does not move with respect to M, the M2 doesn't fall.