# Fun Power Question

1. Nov 10, 2013

### Panphobia

1. The problem statement, all variables and given/known data
Found a pretty fun question on power from one of my friends old physics exams just wanted to see if I was doing it right so far.

3. The attempt at a solution
So the first thing I did was draw a free body diagram and used Newtons Second law so

ƩFx: F - (250 + 0.7v^2) - mgsin10 = ma

Says that the driving force F must overcome the frictional force, so then ma = 0 and F = 250 + 0.7v^2 + mgsin10. A little stumped here, I am trying to find out the velocity, but I have two unknowns and one equation (please don't give me the answer, I want to solve it myself)!

Last edited: Nov 11, 2013
2. Nov 10, 2013

### tiny-tim

Hi Panphobia!

(try using the X2 and X2 buttons just above the Reply box )
I'm confused … the question says a = 1 and v = 27.

3. Nov 10, 2013

### Panphobia

But that is initial, the v is always changing, and in my head I was thinking that there is a maximum velocity because of the air resistance. Oh my was I overthinking that? But anyway I was actually talking about deriving an expression for the power P that is required to drive the wheels. The other part is pretty easy.

Last edited: Nov 10, 2013
4. Nov 10, 2013

### Panphobia

Then F = 1150 + 250 + 0.7(27)^2 + (1150)(9.8)sin10
P = Fv
P = 3867.3*27
= 104 kW or 140 hp

Deriving would just be without numbers right?

5. Nov 10, 2013

### tiny-tim

(i haven't checked your calculations, but …)

yes, those equations look correct
an expression would be without numbers, yes