Question

why are school bags straps flat explain

Answer 1

because if we make it thin like a rope the pressure applied by it on our shoulder increases as pressure=thrust/area so clearly clearly its pressure increases as area decreases so we make it flat because to make pressure low
note-pressure is force applied per unit area means force apllid in every point in the object

Answer 2

to distribute the weight among the shoulder....if u want u can keep one strap on one shoulder and see it will be difficult and also the decrease in area increases the pressure

hope it helps u☺