It is difficult to hold a school bag with a strap made of a thin and strong string because the strap has a small surface area in contact with the hand or shoulder. Since pressure is defined as force divided by area, a thin strap concentrates the force (weight of the bag) on a smaller area, resulting in higher pressure on the hand or shoulder. This high pressure causes discomfort and makes it difficult to hold the bag for a long time. In contrast, broader straps distribute the force over a larger area, reducing the pressure and making it easier to carry the bag comfortably.