This problem, is ridiculous!?

Question

For a human body falling though air in a spread-eagle position, the numerical value of the constant D is about D = 0.250 kg/m.

What value of D is required to make v_t = 42.0 m/s the terminal velocity of a skydiver of mass 85.0 kg? answer in kg/m
Take the free fall acceleration to be g = 9.80 m/s^2.

If the sky diver's daughter, whose mass is 40.0 kg, is falling through the air and has the same D (D=0.25 kg/m) as her father, what is the daughter's terminal speed?

Im not sure if D is drag constant, question does not clarify.

enlighten me if you can solve this

Answer 1

I think that "D" is a combination of the drag coefficient, the denisty of air, and the cross-sectional area of the skydiver. I make that assumtion because they don't provide any of that information, and the units work out. I don't know if it encapsulates the "2" in the terminal velocity formula or not. The numbers look prettier if you assume it does.

So use the formula: Vt^2 = (m x g) / D

Vt = Terminal Velocity
m = mass
g = acceleration of gravity (9.8m/s^2)
D = "the constant"

solve for D:
D = (85 x 9.8)/(42^2)
D = .472


for part 2, solve for Vt:
Vt = sqrt((40 x 9.8)/.0.25)
Vt = 39.6

By the way, the first answer is really really wrong.

Answer 2

Drag might be constant, but the mass to surface area is different. This is what makes a feather fall slower that a brick in the earths atmosphere. Dad has more mass, but a greater surface area. Daughter has less mass but less surface area.

Skydivers can fall together dispite this difference because they can adjust their bodies effective surface area by moving the angle of their arms and legs and by a more horizontal or less horizontal position. This is how the skydivers can bunch up after jumping out of the airplane one at a time. The first one out has fallen farther but assumes a higher drag orientation and falls slower. The following divers assume a lower drag orientation and fall faster until they catch up. This variable doesn't seem to be included in your formula.

Answer 3

I don't know if I can solve the problem or not, but the drag based on gravity would be a constant. The variable would be the surface area of father as compared to the surface area of the daughter which would produce more friction in the father thus slowing his fall slightly.

Answer 4

Terminal velocity is the same for all bodies falling through the same medium.

Else you wouldn't have sky-diving teams now, would you?

Answer 5

She bounces.

Now do your homework.