Drag time of terminal velocity?

[Isawyou0]

Hi all!
How can we know the time and speed of the object before it reachs the terminal veocity, it will reach that, but when?
(he's acceleration decreseases right) thanx

 

[mfb]

If you solve the usual differential equation, the object will never reach the terminal velocity exactly. You can give the time needed until the object reaches 50%, 90%, 99%, ... of the terminal velocity. That will come out of the solution of the differential equation.

If you are just interested in the rough timescale, calculate how long an object without air resistance needs to reach the terminal velocity of the object with resistance.

 

[rcgldr]

For a one dimensional, vertically downwards and uniform gravitational field case, an integral can be solved. Wiki article:

http://en.wikipedia.org/wiki/Free_fall#Uniform_gravitational_field_with_air_resistance

 

[WhatIsGravity]

You could probably get really close as to when and how fast if you're looking at a perfect sphere. Everything else wobbles when it falls, thus it's terminal velocity changes with time.