# Homework Help: TTU Dynamics - Energy Analysis in American Units

1. Nov 23, 2012

### Green Lantern

I am having a problem with dynamics this semester. Our professor gives us "FE-like" questions, which are actually more complicated than FE questions. I can get to where I need to apply the kinetic energy equation but my units look really large and I don't know what to do with W' or how I can get opposing head wind velocity from that.

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

An automobile, weighing 1500 lbf is being driven at 50 mph utilizing 75 hp when
it it struck by head wind that drops its speed by 20%. How fast was the wind?
Ignore all friction but air drag.

2. Relevant equations

F = ma
E = (T+V)
T = (1/2)mv2

3. The attempt at a solution

F = 1500 lbf
v1 = 50 mph
W' = 75 hp
ε = -20%

F = ma: 1500 = m(32.17)
m = 46.63 sl

v2 = v1(1 + ε): v2 = 50(1 - 0.2)
v2 = 40 mph

1 = 1.46 fps
∴v1 = 73 fps and v2 = 58.4 fps

T1 = (1/2)mv12: T1 = (1/2)(46.63)(732)
T1 =

T2 = (1/2)mv22: T2 = (1/2)(46.63)(58.42)
T2 =

The units here don't make sense, since I need to use lbf to get horsepower. Is there a way I can apply force to energy?