Terminal Velocity and Resistive Forces problem

[cheese825]

Homework Statement

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?

Homework Equations

mg = bv
where b is the friction constant
and v is the terminal velocity

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.

 

[Hootenanny]

A number of things could cause your answer to be off.

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.