When a body slides over another body its motion can be analyzed in two ways. Either in the reference
frame of ground or in the reference frame of lower body over which the upper body is sliding. In second
case depending upon the state of motion of lower body the free body diagrams of upper body will
change.
Understanding Pseudo Force: If the lower body is accelerating then relative to lower body, upper body
will accelerate in opposite direction and it experience a force causing this acceleration with respect to
lower body (as seen by an observer on lower body). This force we call Pseudo Force acting on upper
body as it is not an actual force but it appears which causes the acceleration with respect to the lower
body.
So whenever a problem related to motion of a block over another block is analyzed, it can be easily
handled in the reference frame of lower block by considering Pseudo Force on the upper block.
To
understand it see the video – https://youtu.be/fjDMfvpMGB0
Whenever a problem is there on situation of a block sliding over another moving block, its analysis if
done by using the concept of Pseudo Force, the solution will be easier and short.
To develop
applications on how to use Pseudo Force in a variety of problems see the examples given in below videos –