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     The weight of the items in the school bag is supported by the strap.  The strap is pulled down by the weight.

The two sides of the strap are pulled up by the shoulder.  The horizontal part on the shoulder gets pressed inwards due to the weight / force due to the both sides of strap pulling the shoulder down.

The surface under the strap experiences a stress / pressure
       =  Weight of the bag / surface area of strap on the shoulder
The stress or pressure is very high if the strap is a thin one.  The reason is the surface area under the strap is very less.

A person feels pain if the stress or pressure is very high.  The person gets  a piercing or biting feeling.

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