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School bag  has a mass and weight due to gravity.

The weight of the bag is supported by the shoulders.  The shoulder is pressed by the strap downwards with the force equal to the weight of the body.

This force is spread on the area that the strap is in contact with the skin or the body of the person.

Thus, the compressive stress  or, pressure on the body  exerted by the strap
           =  force / area =  weight /area of contact

When the strap is very thin and strong,  the pressure or stress is very high.  Then the skin is pressed hard.  So the body gets pained.  It becomes difficult to hold the bag on the shoulder..  The person feels as if the strap is trying pierce the skin.

When the area of contact with the strap is higher, then the person feels very comfortable.

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