Question

que no. 15..... a block of 2 kg is placed

Answer 1

We need to find the force such that the block can fall freely. Thus the contact force between the block and the wedge should be zero. We can look at this in such a way that what would be the force required to move the wedge (in the direction opposite to the one given) such that the block wouldn't fall. In this case the contact force would be the vector sum of the weight of the block and the pseudo force (I have considered the frame of the wedge which is inertial). Hence the required force would be 100N.

Hope this helps!