Optimizing speed and cost.

1. Feb 6, 2010

fghtffyrdmns

1. The problem statement, all variables and given/known data
The cost of fuel per kilometre for a truck travelling $$v$$ kilometres per hour is given by the equation $$C(v) = \frac{v}{100}+\frac{25}{v}$$. Assume the driver is paid $40/h. What speed would give the lowest cost, including fuel and wagesm for a 1000-km trip? 2. Relevant equations $$C(v) = \frac{v}{100}+\frac{25}{v}$$ 3. The attempt at a solution $$C(v) = \frac{v^2+2500}{100v}$$ I simplified it into one expression. From here, I differentiate and find the minimum speed. I divide$40/h by the speed to get $/km which I can then use to solve again? I do not think this is right. 2. Feb 6, 2010 Mark44 Staff: Mentor You don't want the minimum speed, you want the minimum cost. I agree. The total cost for the trip is the cost for fuel plus the driver's wages. You are given the cost per km as a function of the speed, but you also need to add in the driver's wages, which are also a function of the speed (the faster he drives, the fewer hours, so the less he gets). 3. Feb 6, 2010 fghtffyrdmns That is the part I am stuck at. If I solve the derivative, I would find out the speed which would give the lowest fuel cost per kilometre. 4. Feb 7, 2010 fghtffyrdmns While I tried to do this, I got a local max at x=-50 and local min at x=50. Something does not seem right. 5. Feb 7, 2010 fghtffyrdmns If I am trying to find the minimum cost, I should be using maximum speed which occurs at v=-50. 6. Feb 7, 2010 Mark44 Staff: Mentor What is the function you have for the total cost of the trip? What is it that you are taking the derivative of? You DO NOT want to take the derivative of C(v). 7. Feb 7, 2010 fghtffyrdmns Would I have to multiply the$40/h in to C(v) to get the total cost?

Edit: I know I have to do something with 40.

Last edited: Feb 7, 2010
8. Feb 8, 2010

Staff: Mentor

Yes, you have to do something with the \$40/hr.

Look at what I wrote at the bottom of post #2. If what I have isn't clear, ask questions about it.

9. Feb 8, 2010

fghtffyrdmns

I think I understand this now.

You know that speed is equal to distance/time. Time = distance/speed.

$$t = \frac{1000}{v}$$

Now, to make the total cost function you would write: $$C(t) = (\frac{v}{100}+\frac{25}{v})(1000) + 40t$$

Substitute d/t into the equation, differentiate and solve, you would get how many hours (minimum). Then you can find speed.

10. Feb 8, 2010

Staff: Mentor

That's a little different from what I did, but it seems reasonable, so I think it will work. Notice that your cost function represents fuel cost for the trip + cost of driver's wages, which is what I've been saying you need to work with.