Mechanics: Force-Angle Problem

[morrisj753]

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 =6 b.
(answer: µs > b/a)

Homework Equations

Ff = μ x Fn
F = ma

The Attempt at a Solution

What are the conditions in which the box may tip over? I get what the question is asking of me but I don't really understand how I should first approach it.

 

[cepheid]

morrisj753

See my explanation in post #4 of this thread below, which was about a similar problem:

https://www.physicsforums.com/showthread.php?t=661281

 

[tiny-tim]

hi morrisj753!

What are the conditions in which the box may tip over?

the box will tip if the vertical line through its centre of mass does not go through its area of contact with the surface