When does a rectangular block tip before slipping on an incline?

[usamo42j]

Homework Statement

A uniform rectangular wood block of mass M, with length b and height a, rests on an incline as shown. The incline and the wood block have a coefficient of static friction, μs. The incline is moved upwards from an angle of zero through an angle θ. At some critical angle the block will either tip over or slip down the plane. Determine the relationship between a, b, and μs such that the block will tip over (and not slip) at the critical angle. The box is rectangular, and a ̸= b.

Homework Equations

Fs≤μs*F_n
τ=Fr*sinθ

The Attempt at a Solution

I'm assuming this has to do with torque. There is the normal force doing torque (right?) but that's all I could see. Perhaps gravity also does torque but I'm having trouble seeing how much the radius in the torque formula would be and where does the static friction coefficient come in?

 

[ehild]

Hi usamo,

Start with a picture, showing all the forces and torques.

ehild