Integration of dynamics problem

[fysiikka111]

Homework Statement

A package of mass m is released from an aircraft flying horizontally with a velocity v0. In addition to gravity, a drag force acts on the package given by F=-\lambdav. Find the i and j components of the velocity as a function of the time (t) from when the package was released.

Homework Equations

F=ma

The Attempt at a Solution

Acceleration in x-direction:
F=ma(x)
-\lambdav(x)=ma(x)
a(x)=-\lambdav(x)/m
dv(x)/dt = -\lambdav(x)/m
The solution method then integrates, and gets:
v(x) = v0exp(-\lambdat/m)
I don't know how to do the integration.
Thanks.

 

[Cyosis]

They move all v-s to one side and t-s to the other side.
<br /> \frac{dv}{v}=-\frac{\lambda}{m} dt<br />

You can now easily integrate this equation and solve for v.

 

[fysiikka111]

So,
dv/v = -\lambdadt/m
ln v = -\lambdat/m
v = exp(-\lambdat/m)

But in the answer, where does the extra v0 come from that is before the exponent?

 

[Cyosis]

You forgot the integration constant in combination with v(t=0)=v_0.