Question

What is buoyant force
Why school bags are provided with wide straps

Answer 1

answer of second is in picture

Answer 2

 In physics of an accelerated frame of reference (gravity, centrifugal forces), defining a "downward" direction along the acceleration, buoyancy is the upward force, caused by fluid pressure, that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter. 
As we know that pressure is inversely proportional to area, so an increase in area means decrease in pressure.
School bags are provided with broad straps to increase the surface area in contact with the shoulders and reduce pressure on the shoulders.
If thin straps would be used, then surface area in contact with the shoulders would decrease which will lead to increase in pressure on shoulders.