Hello everyone I am a newbie as well as in physics. I am having a problem in a question as in the attachment. I have learnt that the equation of the static friction (Fs) of an object is equals to the product of the coefficient to the normal force of the object. In this question, let mass of A be M(a) and mass of B be M(b) and the total mass of A and B be M. Assume g is the gravitational acceleration with 9.81ms^-2. However, in this question, what is the reaction force for the Fs between A and B? Is it M times cos θ or just 0.4gM(a)? And after finding the Fs, how does it affect the total acceleration of the objects? Does the kinetic friction have anything to do with the result? I tried to find the Fs by assuming the weight of A times 0.4. Then I consider the force to be horizontal to the plane and hence the force opposing the motion (Fb) would be Fs cos θ. After that the resultant force will equals to Mg sin θ - Fb. But that gave me a negative answer. So where is the error?