Simple Incline Problem: Acceleration of a Block on a Wedge - Solution Attempt

[AlchemistK]

Homework Statement

A 6kg block "B" rests as shown(refer attachment) on the upper surface of a 15 Kg wedge "A". Neglecting friction, determine immediately after the system is released from rest,
a)The acceleration of A
b)The acceleration of B relative to A.

Homework Equations

F = ma

The Attempt at a Solution

For the first part,

The force on A due to gravity, parallel to the incline will be Mg sin30° = 75N
And the force of the block B on A is mg= 60N, and the component parallel to the incline will be 60 sin30° = 30N, so the total force parallel to the incline is 30+75= 105N
Thus, a=Net force/ total mass = 105/(15+6) = 5 m/s^2 .
The answer should be 6.63 m/s^2. Where have i gone wrong?

 

[Spinnor]

I think you might have two problems, first you use the total mass when calculating the acceleration but I think it should only be the 15kg mass, the two blocks accelerate differently. Secondly I think the force of the upper block acting on the lower block is reduced because the upper block accelerates downward. There is a geometric relationship between the accelerations. See,