What is a pseudo force? Explain with the help of an appropriate example.

Solve the following problem using the concept of pseudo force:

Two rectangular blocks are stacked one over the other on a table. The lower and upper blocks have masses of 25 kg and 5 kg respectively. The coefficient of kinetic friction between the lower block and table is 0.3 and that between the two blocks is 0.6. A pulling force is applied to the lower block by means of a mass less rope attached to it.

a) Draw the free body diagram of the lower block "as seen" from the frame of reference of the table (or ground).

b) Draw the free body diagram of the upper block "as seen" i) from the frame of reference of the table (or ground), ii) from the frame of reference of the lower block.

c) At what acceleration of the lower block does the upper block start slipping relative to the lower block?

d) How much pulling force must be applied to the lower block to give it the acceleration calculated at c) above?