Then, the force analysis of the object A shows that there may be a static friction force F static between the ground and A due to the gravity Mg, the supporting force FN of the ground, the pressure N' of B against A, and the friction F' of B against A, so it is assumed to be static at first and to the right, as shown in the figure.
When the object B slides at a uniform speed, according to the point force balance condition, there are
mgsinθ-f=0①
N-mgcosθ=0②
When object b slides down at an accelerated speed, there are
mgsinθ>f③
N-mgcosθ=0④
When object B slows down and slides down, there are
mgsinθ