Gravitational and kinetic energy problem

[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?
Ty in advance!

[arildno]

Why should you want joules for??

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

[lzh]

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

[arildno]

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

[lzh]

velocity!
i see, ty

[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

[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?

[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?

[arildno]

Correct!:smile:

[lzh]

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

[arildno]

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

How fortunate for the hw service! :smile: