1. The problem statement, all variables and given/known data: block M slides down on frictionless incline .Find the minimum co efficient of friction so that m does not slide with respect to M. 2. Relevant equations:acceleration of the system=total driving force/total mass: 3. The attempt at a solution m does not slide with respect to M means there should not be kinetic friction rather static friction.both small and big block will have same acceleration. acceleration of the system=total driving force/total mass :total driving force=(m+M)g sin theta acceleration of the system=(m+M)g sin theta/m+M =g sin theta force responsible for acceleration of small block would be force of static friction between two blocks. Hence force of static friction between two blocks=mass of small block i.e "m" multiplied by g sin theta but this force of static friction between two blocks is also equal to or smaller than μ multiplied by normal force on small block i. mg force of static friction ≤ μmg force of static friction/mg≤ μ m× g sin theta= force of static friction Hence m× g sin theta/mg ≤ μ theta= 37 degrees sin theta ≤ μ sin 37 degrees ≤ μ 0.601 ≤ μ but the correct answer should be 0.75 ≤ μ where am I going wrong?