Mass not sliding on a smooth, accelerating base

[Karol]

Homework Statement

Mass m lays on the smooth triangle of mass M. what is the acceleration of M so that m will stay in place.

Homework Equations

Newton's second law: $F=ma$

The Attempt at a Solution

$$\tan\alpha=\frac{ma}{mg}\;\rightarrow\; a=g\tan\alpha$$

 

[rpthomps]

My suggestion is to determine the acceleration of the block if the ramp is not moving first, down the slope.

 

[Nathanael]

Looks right to me, is your answer supposed to be wrong?

 

[Karol]

Looks right to me, is your answer supposed to be wrong?

It's one of the possible answers but the other answers are complicated and include M. it's logical that my answer is correct since it's a short quiz, not a long exam