How Long Does It Take a Raindrop to Reach 63% of Its Terminal Velocity?

[peweee17]

Homework Statement

The terminal velocity of a 2×10−5 kg raindrop is about 7m/s. Assuming a drag force Fd= - bv,
Assuming a drag force determine the time required for such a drop, starting from rest, to reach 63% of terminal velocity.

Homework Equations

Fd=-bv
Sum of F-ma

The Attempt at a Solution

I use Fd=-bv to solve for b then used Fd-Fg=ma and reduced that to dv/dt=-g-(bv/m) but don't know how to determine time.

 

[cristo]

You need to show some work before we can help you: what have you tried?