Problem 4: A weather balloon has an absolute-pressure sensor attached. On the ground the sensor reads Pa = 1.01x105 Pa. At a height of h = 975 m, the sensor reads PB = 8.9x104 Pa. = = 50% Part (a) Assuming air is an incompressible fluid, enter an expression for an estimate of the density of air, in terms of the defined quantities and the acceleration due to gravity, g. 50% Part (b) Calculate the density of air, in kilograms per cubic meter, assuming it is an incompressible fluid.