Homework Help: Help integrating

1. Nov 23, 2009

Punkyc7

1. The problem statement, all variables and given/known data
Derive an equation for an objects velocity as a function of time

2. Relevant equations

i have that a=-(kvo/m)

3. The attempt at a solution
so i get dv/v=-(k/m)dt then i get
1/v= -kt/m +C and then im stuck

2. Nov 23, 2009

XanziBar

It's been awhile, but I think the integral of dv/v is the natural log of v. Then you can probably exponentiate both sides

3. Nov 23, 2009

Punkyc7

so
lnv= ln(-kt/m +C)
then
e^lnv=e^ln(-kt/m +C)
so is this right
v=e^ln(-kt/m +C)

4. Nov 23, 2009

XanziBar

you only have the 1/v on the left side:

lnv= (-kt/m +C)

e^lnv=e^(-kt/m +C)
so is this right
v=e^(-kt/m +C)

yeah, that seems about right. I'm a little worried about the units, maybe you can do something with that k or C...

5. Nov 24, 2009

rl.bhat

a=-(kvo/m)
The above relation represent rocket equation where vo represents the velocity of escaping of gas which is constant an k represents dm/dt, the mass of fuel ejected per unit time. It is also constant. Here mass of the fuel is varying with time.
So you can find the velocity of the object with respect to mass rather than the time.
a = dv/dt = - (dm/dt)vo/m
dv = -vo(dm/m). To find the velocity take the integration between the limits m = Mo to m = M.