Doubt from Friction

solved
sm
friction
relative-motion
easy
#1

for 2, this is soln given but if there is rel. motion inn btwn block and plank then how will they have same accl????

#2

When there is realtive motion, F = pseudo force on m + friction.

#3

#4

This is the point of slipping ,acceleration of both bodies till now is same so friction did not act
(Here we solve question in the ground frame)