Terminal Velocity and Resistive Forces problem

1. Nov 16, 2008

cheese825

1. The problem statement, all variables and given/known data
A sky diver of mass 81.5 kg jumps from a slow-moving aircraft and reaches a terminal speed of 50.0 m/s.
(a) What is the acceleration of the sky diver when her speed is 30.0 m/s?

2. Relevant equations
mg = bv
where b is the friction constant
and v is the terminal velocity

3. The attempt at a solution
b = (81.5)(9.8) / 50
b = 15.97

(81.5)(9.8) - (15.97)(30) = remaining force, or 319.6 Newtons
319.6 N / 81.5 = 3.921

but it seems, 3.921 is not correct. I must be missing something big.
Any help would be appreciated.

Last edited: Nov 16, 2008
2. Nov 16, 2008

Hootenanny

Staff Emeritus

Firstly, if you are entering your answers online be careful with your significant figures - online marking software can be very pedantic about such things.

Secondly, were you explicitly told that the drag force was a linear function of velocity because for large objects moving faster than a few m/s the drag force is usually a quadratic function of velocity.