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

Hint: In this particular question, we will check the dependence of pressure on area.
As we know that the pressure is the ratio of force and area. It means that pressure is directly proportional to the force and inversely proportional to the area.
\[P \propto \dfrac{F}{A}\]

Complete step by step solution:
As we know that pressure is inversely proportional to the area.
\[P \propto \dfrac{1}{A}\]
Here P is the pressure and A is the area. If the area is smaller, then the pressure will be more and vice-versa.
In this particular problem, It is difficult to hold a strap that is made up of thin and strong string because the area under the strap is small. So, more pressure will be exerted by the strap on our shoulders. Due to this large pressure that is exerted by the strap there might be chances of getting hurt.

Note:
As we know that pressure depends on the force applied and the area on which that force acts. SI unit of pressure is \[N/{m^{_2}}\].
We also see bags with broad straps, in that case for avoiding the pressure on the shoulders they are made with the broad straps, so that the weight is distributed all over the shoulder and we can lift more stuff for a longer time without feeling so much pain.