Question

a load on thief''s back doesn't apply any force on him when he jumps from the upper story of a house

Answer 1

Both the thief and load falls down with same acceleration due to gravity.* As there is no relative motion between them, the load is considered to bein rest when compared to the thief. so, there is no possibility of force applied on the thief due to zero relative accelation between them.
F = m arel      
= m × 0 = 0