Terminal velocity/drag

[bigboss]

Homework Statement

The fastest recorded skydive was by an Air Force officer who jumped from a helium balloon at an elevation of 103000 ft, three times higher than airliners fly. Because the density of air is so small at these altitudes, he reached a speed of 614 mph at an elevation of 90000 ft, then gradually slowed as the air became more dense. Assume that he fell in the spread-eagle position and that his low-altitude terminal speed is 125 mph. Use this information to determine the density of air at 90000 ft.

Homework Equations

V=sqrt(4W/pA) w= weight, p=coefficent, a = area ... D=.25pv^2A

The Attempt at a Solution

no idea what to do...

 

[LowlyPion]

Consider the two equations at the two altitudes for terminal velocity.

Fdrag = m*g = 1/2*C_d*p*A*v²

Since the weight is the same ...

1/2*C_d*p₉₀*A*V₉₀² = 1/2*C_d*p_o*A*V_o²

p₉₀*V₉₀² = p_o*V_o²

 

[bigboss]

i got .054, however that did not work... i used 1.29 as the p for the low altitude

 

[LowlyPion]

i got .054, however that did not work... i used 1.29 as the p for the low altitude

What units do they want the answer in?

Imperial or metric?

At 20 °C and 101.325 kPa, dry air has a density of 1.2041 kg/m³.
At 70 °F and 14.696 psia, dry air has a density of 0.074887 lbm/ft³.

 

[bigboss]

.054 kg/m^3.