Gravitational and kinetic energy problem

1. Jan 20, 2007

lzh

1. The problem statement, all variables and given/known data
Given: 1 hp = 746 W and 1 m/s =2.237 mi/h.
When an automobile moves with constant
velocity, the power developed in a certain en-
gine is 85.4 hp.
What total frictional force acts on the car
at v = 72.8 mi/h? Answer in units of N.

2. Relevant equations
Kinetic Energy=.5mv^2
Disspated energy=F*displacement <-the f would be the frictional force.
W=change in energy(J)/change in time(s)
3. The attempt at a solution
i first converted 85.4 hp to W: 63708.4W
then the v to m/s: 32.54m/s

i would like to find the amount of joules in 63708.4W, but I dont know the time. So I probably need to find a way to cancel it.

also, I don't know the mass of the car...nor the displacement. I'm not sure of what to do with so many unknowns.
Can someone point my problems out?

2. Jan 20, 2007

arildno

Why should you want joules for??

Instead, how are the quantities power, force and velocity related?

3. Jan 20, 2007

lzh

power=Force*displacement/time
after some simplification
I'm starting to see something though
edit:i forgot to type /t

Last edited: Jan 20, 2007
4. Jan 20, 2007

arildno

You have constant velocity. What is therefore displacement/time equal to?

Last edited: Jan 20, 2007
5. Jan 20, 2007

lzh

velocity!
i see, ty

6. Jan 20, 2007

lzh

so that means Force*v=63708W
substituting v i get:
F=1957.84N
but thats not the frictional force, I'm going to solve for that next

7. Jan 20, 2007

arildno

So, what you have found is the force the engine provides on the system, agreed?

Now, how much acceleration does the car experience if it moves with constant velocity?

8. Jan 20, 2007

lzh

yeah thats the force of the engine.
car at constand velocity has 0 accelaeration
so at constant velocity, the force of friction must be equal to force of engine?
so F=1957.84N is correct?

Last edited: Jan 20, 2007
9. Jan 20, 2007

arildno

Correct!

10. Jan 20, 2007

lzh

ty!
it was correct, according to my utexas's hw service

11. Jan 20, 2007

arildno

How fortunate for the hw service!