# Velocity as a function of distance [v(x)]

1. May 3, 2012

### BitterX

1. The problem statement, all variables and given/known data
a body with mass M moves across a plane with friction

friction constant:
$\mu = \lambda x^2$

the body starts at x=0
with velocity v0

find at what x
the body stops
and what was the velocity half way there.

2. Relevant equations

$v^2=v_0^2+2a\Delta x$

3. The attempt at a solution

obviously,
$F(x)=mg\mu = mg\lambda x^2$
so
$a(x)=g \lambda x^2$

so in the equation $v^2=v_0^2+2a\Delta x$
I get
$v^2=v_0^2+2g\lambda x^3$

the Question is, can I use this equation? the acceleration is not constant and this equation
depend on the fact that $x=v_0t+ \frac{a}{2}t^2$
and $v=v_0+at$
(and it's not true for non-constant acceleration)

if I cant, how can I integrate the acceleration?
or how do I get v(x)?

Thanks.

EDIT:
I used $a= v\frac{dv}{dx}$
therefore
$vdv=adx$

$\int_{v_0}^{v(x)}{vdv} = g\lambda \int_{0}^{x}{x^2}$

$\frac{1}{2} ( v(x)^2- v_0^2) =\frac{1}{3} g\lambda x^3$

$v(x)^2=v_0^2+\frac{2}{3}g\lambda x^3$

does that seem right?

Last edited: May 3, 2012
2. May 3, 2012

### gulfcoastfella

I don't see anything wrong with your edited solution.

3. May 4, 2012

### ehild

Does the friction increase speed?
Remember that velocity, acceleration and force are all vectors. You need to use proper signs with them.

ehild

4. May 4, 2012

### BitterX

ah, of course... it's with a minus :)
on paper I actually did it with a minus. Thanks for pointing it out though!