Free body diagram(its simple but i don't know why i can't understand it)

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Homework Statement

If a 4kg block is placed on top of a 5kg block which placed on a friction less plane and a force is applied on the lower 5kg block ,For what maximum value of force will the top block not slide ?
μ=0.036

Homework Equations

The Attempt at a Solution

The maximum static friction that the top block can experience is f_max=μ mg=12 N
But the force acts on the lower block so what will be the force on the upper block that the friction will counter?
It would be nice to get a free body diagram.

 

[pgardn]

Homework Statement

If a 4kg block is placed on top of a 5kg block which placed on a friction less plane and a force is applied on the lower 5kg block ,For what maximum value of force will the top block not slide ?
μ=0.036

Homework Equations

The Attempt at a Solution

The maximum static friction that the top block can experience is f_max=μ mg=12 N
But the force acts on the lower block so what will be the force on the upper block that the friction will counter?
It would be nice to get a free body diagram.

Would you agree that the 5 kg block will have to accelerate if a force is applied horizontally?

 

[ehild]

The frictional acts between the blocks: it is a force of interaction that opposes the relative motion of both blocks. Try to draw the free body diagram separately for both blocks.

What force acts on the upper block? What would happen to it without friction?

By the way, how did you get fmax=μ mg=12 N with μ=0.036?

ehild