# Homework Help: Thermodynamics question 1 (question about mass)

1. Sep 18, 2008

1. The problem statement, all variables and given/known data

Alrighty then

I still cannot get used to texts that use lbf (lb force) and lbm (lb mass) and just plain old lb.
Metric is bomb.

I am trying to find the power needed too overcome drag and rolling resistance forces from this given info.

$F_r=f\cdot W$=Rolling resistance force
$F_d=C_dA\frac{1}{2}\rho v^2$=Drag force
Power=$P=F\cdot V$
Area=A=23.3 sq ft
$C_d=.34$
$f=.02$
$\rho=.08\frac{lb}{ft^3}$
$v=55\frac{mi}{h}$
weight=$w=3550lbf$ <------how do I convert this term properly? I think I NEED TO

So, if P=F*V Total power is the sum of this product.

$$P=(\sum F)\cdot V$$

$$\Rightarrow P=(f*w+C_dA\frac{1}{2}\rho v^2)v$$

$$\Rightarrow P=(.02*3550lbf+.34*23.3ft^2*\frac{1}{2}*.08\frac{lb}{ft^3}*55^2\frac{mi^2}{h^2}*5280^2\frac{ft^2}{mi^2}*\frac{h^2}{3600^2s^s})*(55\frac{mi}{h}*\frac{5280ft}{mi}*\frac{h}{3600s})$$

I cannot tell what to do with that "lbf" ? I know that tyhis is really simple, but I am not used to these units

Little help? Thanks!!

EDIT: Is a lbf just a lbm*ft/s^2 ? I am assuming that density is given in lbm even though it does not specify...

Last edited: Sep 18, 2008
2. Sep 18, 2008