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Terminal velocity/drag

  • Thread starter bigboss
  • Start date
  • #1
28
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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...
 

Answers and Replies

  • #2
LowlyPion
Homework Helper
3,090
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Consider the two equations at the two altitudes for terminal velocity.

Fdrag = m*g = 1/2*Cd*p*A*v2

Since the weight is the same ...

1/2*Cd*p90*A*V902 = 1/2*Cd*po*A*Vo2

p90*V902 = po*Vo2
 
  • #3
28
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i got .054, however that did not work... i used 1.29 as the p for the low altitude
 
  • #4
LowlyPion
Homework Helper
3,090
4
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?

Wikipedia said:
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.
 
  • #5
28
0
.054 kg/m^3.
 

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