Question

Why does a porter bend forward while carrying a sack of rice on his back

Answer 1

because of the weight of the sack.

Answer 2

Porter bends forward while carrying a sack of rice on his back just to increase the area of contact between his back and sack of rice as this will reduce the pressure exerted by the sack on his body.

$pressure = \frac{force}{area}$

this shows that pressure is inversely proportional to area.

Hence when he increases the area pressure automatically gets reduced.

Hope it helps!!!