# Homework Help: Force needed to keep block from moving on frictionless triangular block

1. Sep 26, 2008

### Strukus

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

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

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

3. 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.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 27, 2008

### tiny-tim

Welcome to PF!

Hi Strukus! Welcome to PF!
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?