Question

why is it difficult to hold a school bag having a strap made of a thin and strong string

Answer 1

This is because area is inveresely proportional to pressure,so if strap is thin then area is less and pressure on our shoulder increases and we suffer from pain.
Hope it hlps u

Answer 2

This is because when the area of the straps of the bag decreases, the pressure on your shoulders will increase, since area is inversely proportional to pressure.
Hope my answer helped.
Thanks! ☺