# Terminal velocity in a viscous medium

1. Oct 15, 2007

### shiri

A 11.00 kg object starting from rest falls through a viscous medium and experiences a resistive force R = -bv, where v is the velocity of the object. The object reaches one half its terminal speed in 5.84 s.

(a) Determine the terminal speed.
82.7 m/s

(b) At what time is the speed of the object three-fourths the terminal speed?
11.7 s

I couldn't figure out on this question. can someone help me out, i'll be appreciated
(c) How far has the object traveled in the first 5.84 s of motion?

2. Oct 16, 2007

### Hootenanny

Staff Emeritus
Can you write an equation of motion for the object?

3. Oct 16, 2007

### shiri

was it v = [Initial velocity] + [(1/2)at^2]?

4. Oct 17, 2007

### Hootenanny

Staff Emeritus
That equation is only valid for constant acceleration, but is the acceleration constant in this case?

5. Oct 17, 2007

### shiri

well what i got for this question is x=d=mgt/b + m^2g/b^2 * (exp(-bt/m) - 1)=483m
but I don't understand why this is a wrong answer so can you tell me why?