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

Solution:
It is difficult to hold a school bag having a thin strap because the pressure on the shoulders is quite large. This is because the pressure is inversely proportional to the area on which the force acts. The smaller is the area, the larger will be the pressure on the surface. In the case of a thin strap, the contact area is very small. Hence, the pressure exerted on the shoulder is very large.