Force needed to keep block from moving on frictionless triangular block

[Strukus]

Homework Statement

This is straight from the book.

A small block of mass m rests on the sloping side of a triangular block of mass M which itself rests on a horizontal table as shown in Fig. 4-50. Assuming all surfaces are frictionless, determine the force F that must be applied to M so that m remains in a fixed position relative to M (that is, m doesn't move on the incline).

(I forgot to draw the surface the triangular block is on and sorry for the big picture!)

The answer is:

Spoiler

(m + M) $$\ast$$ g$$\ast$$ tan($$\phi$$)

Homework Equations

Force along the x-axis: m $$\ast$$ g $$\ast$$ sin($$\phi$$)
Force along the y-axis: m $$\ast$$ g $$\ast$$ cos($$\phi$$)

The Attempt at a Solution

I know that there must be an equal and opposite force along the incline but I have no clue how to approach this problem.

 

[tiny-tim]

Welcome to PF!

Hi Strukus! Welcome to PF!

Force along the x-axis: m $$\ast$$ g $$\ast$$ sin($$\phi$$)
Force along the y-axis: m $$\ast$$ g $$\ast$$ cos($$\phi$$)

No, that's the wrong approach …

only one of those equations is correct …

start again … you know the vertical acceleration of m is zero, so what is N (the normal reaction force ) …

and then what is the horizontal component of N?